Please address the comments from this code review:

## Overall Comments
- Extract the shared file-reading and URL-handling logic in _read_attachment_data, _is_url, and _download_from_url into a common utility to eliminate duplication across backends and the service layer.
- Consolidate the duplicate _store_attachments implementations in FakeFileBackend and FakeAsyncIOFileBackend into a single base method to adhere to DRY principles.
- In the async service layer, the synchronous requests.get call in _download_from_url can block the event loop—consider using an async HTTP client or offloading the download to a thread pool.

## Individual Comments

### Comment 1
<location> `vintasend/tests/test_services/test_notification_attachments.py:217-220` </location>
<code_context>
+        retrieved_data = stored_attachment.get_file_data()
+        assert retrieved_data == test_data
+
+    def test_store_attachments_with_url(self):
+        """Test storing attachments from URLs (using fake download)"""
+        url = "http://example.com/document.pdf"
+        attachment = NotificationAttachment(
+            filename="document.pdf",
+            file=url,
+        )
+
+        stored = self.backend._store_attachments([attachment])
+
+        assert len(stored) == 1
+        stored_attachment = stored[0]
+        assert stored_attachment.filename == "document.pdf"
+
+        # Should contain fake downloaded content
+        retrieved_data = stored_attachment.get_file_data()
+        assert b"Downloaded content from" in retrieved_data
+
+    def test_store_attachments_empty_list(self):
</code_context>

<issue_to_address>
**suggestion (testing):** Consider adding a test for unsupported URL schemes.

Adding a test for an unsupported URL scheme, such as 'ftp://', will help verify that the backend correctly raises a ValueError or handles the input as expected.

```suggestion
    def test_store_attachments_unsupported_url_scheme(self):
        """Test storing attachments with unsupported URL scheme should raise ValueError"""
        url = "ftp://example.com/document.pdf"
        attachment = NotificationAttachment(
            filename="document.pdf",
            file=url,
        )
        with pytest.raises(ValueError):
            self.backend._store_attachments([attachment])

    def test_store_attachments_empty_list(self):
        """Test storing empty attachment list"""
        stored = self.backend._store_attachments([])
        assert stored == []
```
</issue_to_address>

### Comment 2
<location> `vintasend/tests/test_services/test_notification_attachments.py:701-710` </location>
<code_context>
+    def test_notification_attachment_file_types(self):
</code_context>

<issue_to_address>
**suggestion (testing):** Missing test for Path object as file input.

Please add a test case using pathlib.Path to verify support for Path objects as file inputs.
</issue_to_address>

### Comment 3
<location> `vintasend/tests/test_services/test_notification_attachments.py:721-730` </location>
<code_context>
+    def test_content_type_detection_accuracy(self):
</code_context>

<issue_to_address>
**suggestion (testing):** Consider adding a test for files with no extension.

Please add a test for filenames without an extension to confirm that 'application/octet-stream' is used as the default content type.
</issue_to_address>

### Comment 4
<location> `vintasend/tests/test_services/test_notification_attachments.py:179-177` </location>
<code_context>
+        retrieved_data = stored_attachment.get_file_data()
+        assert retrieved_data == test_data
+
+    def test_store_attachments_with_file_like_object(self):
+        """Test storing attachments with file-like objects"""
+        test_data = b"File-like object content"
+        file_obj = io.BytesIO(test_data)
+
+        attachment = NotificationAttachment(
+            filename="filelike.txt",
+            file=file_obj,
+        )
+
+        stored = self.backend._store_attachments([attachment])
+
+        assert len(stored) == 1
+        stored_attachment = stored[0]
+        assert stored_attachment.size == len(test_data)
+
+        # Verify file data
+        retrieved_data = stored_attachment.get_file_data()
+        assert retrieved_data == test_data
+
+    def test_store_attachments_with_url(self):
</code_context>

<issue_to_address>
**suggestion (testing):** Consider adding a test for io.StringIO as file input.

Please add a test that uses io.StringIO to ensure attachments from StringIO objects are processed and encoded to bytes as expected.
</issue_to_address>

### Comment 5
<location> `vintasend/tests/test_services/test_notification_attachments.py:97-102` </location>
<code_context>

</code_context>

<issue_to_address>
**issue (code-quality):** Avoid loops in tests. ([`no-loop-in-tests`](https://docs.sourcery.ai/Reference/Rules-and-In-Line-Suggestions/Python/Default-Rules/no-loop-in-tests))

<details><summary>Explanation</summary>Avoid complex code, like loops, in test functions.

Google's software engineering guidelines says:
"Clear tests are trivially correct upon inspection"
To reach that avoid complex code in tests:
* loops
* conditionals

Some ways to fix this:

* Use parametrized tests to get rid of the loop.
* Move the complex logic into helpers.
* Move the complex part into pytest fixtures.

> Complexity is most often introduced in the form of logic. Logic is defined via the imperative parts of programming languages such as operators, loops, and conditionals. When a piece of code contains logic, you need to do a bit of mental computation to determine its result instead of just reading it off of the screen. It doesn't take much logic to make a test more difficult to reason about.

Software Engineering at Google / [Don't Put Logic in Tests](https://abseil.io/resources/swe-book/html/ch12.html#donapostrophet_put_logic_in_tests)
</details>
</issue_to_address>

### Comment 6
<location> `vintasend/tests/test_services/test_notification_attachments.py:712-719` </location>
<code_context>

</code_context>

<issue_to_address>
**issue (code-quality):** Avoid loops in tests. ([`no-loop-in-tests`](https://docs.sourcery.ai/Reference/Rules-and-In-Line-Suggestions/Python/Default-Rules/no-loop-in-tests))

<details><summary>Explanation</summary>Avoid complex code, like loops, in test functions.

Google's software engineering guidelines says:
"Clear tests are trivially correct upon inspection"
To reach that avoid complex code in tests:
* loops
* conditionals

Some ways to fix this:

* Use parametrized tests to get rid of the loop.
* Move the complex logic into helpers.
* Move the complex part into pytest fixtures.

> Complexity is most often introduced in the form of logic. Logic is defined via the imperative parts of programming languages such as operators, loops, and conditionals. When a piece of code contains logic, you need to do a bit of mental computation to determine its result instead of just reading it off of the screen. It doesn't take much logic to make a test more difficult to reason about.

Software Engineering at Google / [Don't Put Logic in Tests](https://abseil.io/resources/swe-book/html/ch12.html#donapostrophet_put_logic_in_tests)
</details>
</issue_to_address>

### Comment 7
<location> `vintasend/tests/test_services/test_notification_attachments.py:736-742` </location>
<code_context>

</code_context>

<issue_to_address>
**issue (code-quality):** Avoid loops in tests. ([`no-loop-in-tests`](https://docs.sourcery.ai/Reference/Rules-and-In-Line-Suggestions/Python/Default-Rules/no-loop-in-tests))

<details><summary>Explanation</summary>Avoid complex code, like loops, in test functions.

Google's software engineering guidelines says:
"Clear tests are trivially correct upon inspection"
To reach that avoid complex code in tests:
* loops
* conditionals

Some ways to fix this:

* Use parametrized tests to get rid of the loop.
* Move the complex logic into helpers.
* Move the complex part into pytest fixtures.

> Complexity is most often introduced in the form of logic. Logic is defined via the imperative parts of programming languages such as operators, loops, and conditionals. When a piece of code contains logic, you need to do a bit of mental computation to determine its result instead of just reading it off of the screen. It doesn't take much logic to make a test more difficult to reason about.

Software Engineering at Google / [Don't Put Logic in Tests](https://abseil.io/resources/swe-book/html/ch12.html#donapostrophet_put_logic_in_tests)
</details>
</issue_to_address>

### Comment 8
<location> `vintasend/services/notification_adapters/stubs/fake_adapter.py:38` </location>
<code_context>
    def send(self, notification: "Notification | OneOffNotification", context: "NotificationContextDict") -> None:
        self.template_renderer.render(notification, context)

        # Capture attachment information for testing
        attachment_info = []
        for attachment in notification.attachments:
            attachment_info.append({
                'id': str(attachment.id),
                'filename': attachment.filename,
                'content_type': attachment.content_type,
                'size': attachment.size,
                'is_inline': attachment.is_inline,
                'description': attachment.description,
                'checksum': attachment.checksum,
            })

        self.sent_emails.append((notification, context, attachment_info))

</code_context>

<issue_to_address>
**issue (code-quality):** Convert for loop into list comprehension ([`list-comprehension`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/list-comprehension/))
</issue_to_address>

### Comment 9
<location> `vintasend/services/notification_adapters/stubs/fake_adapter.py:69` </location>
<code_context>
    async def send(self, notification: "Notification | OneOffNotification", context: "NotificationContextDict") -> None:
        self.template_renderer.render(notification, context)

        # Capture attachment information for testing
        attachment_info = []
        for attachment in notification.attachments:
            attachment_info.append({
                'id': str(attachment.id),
                'filename': attachment.filename,
                'content_type': attachment.content_type,
                'size': attachment.size,
                'is_inline': attachment.is_inline,
                'description': attachment.description,
                'checksum': attachment.checksum,
            })

        self.sent_emails.append((notification, context, attachment_info))

</code_context>

<issue_to_address>
**issue (code-quality):** Convert for loop into list comprehension ([`list-comprehension`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/list-comprehension/))
</issue_to_address>

### Comment 10
<location> `vintasend/services/notification_backends/stubs/fake_backend.py:301` </location>
<code_context>
    def _read_attachment_data(self, file) -> bytes:
        """Read file data from various file-like object types"""
        from pathlib import Path

        if isinstance(file, bytes):
            return file
        elif isinstance(file, str):
            if self._is_url(file):
                return self._download_from_url(file)
            else:
                # Read from file path
                with open(file, 'rb') as f:
                    return f.read()
        elif isinstance(file, Path):
            with open(file, 'rb') as f:
                return f.read()
        elif hasattr(file, 'read'):
            current_pos = file.tell() if hasattr(file, 'tell') else 0
            if hasattr(file, 'seek'):
                file.seek(0)
            data = file.read()
            if hasattr(file, 'seek'):
                file.seek(current_pos)
            if isinstance(data, str):
                return data.encode('utf-8')
            return data
        else:
            raise ValueError(f"Unsupported file type: {type(file)}")

</code_context>

<issue_to_address>
**issue (code-quality):** We've found these issues:

- Remove unnecessary else after guard condition ([`remove-unnecessary-else`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/remove-unnecessary-else/))
- Extract code out into method ([`extract-method`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/extract-method/))
- Lift code into else after jump in control flow ([`reintroduce-else`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/reintroduce-else/))
- Replace if statement with if expression ([`assign-if-exp`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/assign-if-exp/))
</issue_to_address>

### Comment 11
<location> `vintasend/services/notification_backends/stubs/fake_backend.py:548` </location>
<code_context>
    def _read_attachment_data(self, file) -> bytes:
        """Read file data from various file-like object types"""
        from pathlib import Path

        if isinstance(file, bytes):
            return file
        elif isinstance(file, str):
            if self._is_url(file):
                return self._download_from_url(file)
            else:
                # Read from file path
                with open(file, 'rb') as f:
                    return f.read()
        elif isinstance(file, Path):
            with open(file, 'rb') as f:
                return f.read()
        elif hasattr(file, 'read'):
            current_pos = file.tell() if hasattr(file, 'tell') else 0
            if hasattr(file, 'seek'):
                file.seek(0)
            data = file.read()
            if hasattr(file, 'seek'):
                file.seek(current_pos)
            if isinstance(data, str):
                return data.encode('utf-8')
            return data
        else:
            raise ValueError(f"Unsupported file type: {type(file)}")

</code_context>

<issue_to_address>
**issue (code-quality):** We've found these issues:

- Remove unnecessary else after guard condition ([`remove-unnecessary-else`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/remove-unnecessary-else/))
- Extract code out into method ([`extract-method`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/extract-method/))
- Lift code into else after jump in control flow ([`reintroduce-else`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/reintroduce-else/))
- Replace if statement with if expression ([`assign-if-exp`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/assign-if-exp/))
</issue_to_address>

### Comment 12
<location> `vintasend/services/notification_service.py:185` </location>
<code_context>
    def _read_file_data(self, file) -> bytes:
        """Read file data from various file-like object types"""
        from pathlib import Path

        if isinstance(file, str):
            if self._is_url(file):
                return self._download_from_url(file)
            else:
                # Read from file path
                with open(file, 'rb') as f:
                    return f.read()
        elif isinstance(file, Path):
            with open(file, 'rb') as f:
                return f.read()
        elif hasattr(file, 'read'):
            current_pos = file.tell() if hasattr(file, 'tell') else 0
            if hasattr(file, 'seek'):
                file.seek(0)
            data = file.read()
            if hasattr(file, 'seek'):
                file.seek(current_pos)
            if isinstance(data, str):
                return data.encode('utf-8')
            return data
        else:
            raise ValueError(f"Unsupported file type: {type(file)}")

</code_context>

<issue_to_address>
**issue (code-quality):** We've found these issues:

- Merge duplicate blocks in conditional ([`merge-duplicate-blocks`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/merge-duplicate-blocks/))
- Extract code out into method ([`extract-method`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/extract-method/))
- Lift code into else after jump in control flow ([`reintroduce-else`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/reintroduce-else/))
- Replace if statement with if expression ([`assign-if-exp`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/assign-if-exp/))
- Remove redundant conditional ([`remove-redundant-if`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/remove-redundant-if/))
</issue_to_address>

### Comment 13
<location> `vintasend/services/notification_service.py:741` </location>
<code_context>
    def _read_file_data(self, file) -> bytes:
        """Read file data from various file-like object types"""
        from pathlib import Path

        if isinstance(file, str):
            if self._is_url(file):
                return self._download_from_url(file)
            else:
                # Read from file path
                with open(file, 'rb') as f:
                    return f.read()
        elif isinstance(file, Path):
            with open(file, 'rb') as f:
                return f.read()
        elif hasattr(file, 'read'):
            current_pos = file.tell() if hasattr(file, 'tell') else 0
            if hasattr(file, 'seek'):
                file.seek(0)
            data = file.read()
            if hasattr(file, 'seek'):
                file.seek(current_pos)
            if isinstance(data, str):
                return data.encode('utf-8')
            return data
        else:
            raise ValueError(f"Unsupported file type: {type(file)}")

</code_context>

<issue_to_address>
**issue (code-quality):** We've found these issues:

- Merge duplicate blocks in conditional ([`merge-duplicate-blocks`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/merge-duplicate-blocks/))
- Extract code out into method ([`extract-method`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/extract-method/))
- Lift code into else after jump in control flow ([`reintroduce-else`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/reintroduce-else/))
- Replace if statement with if expression ([`assign-if-exp`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/assign-if-exp/))
- Remove redundant conditional ([`remove-redundant-if`](https://docs.sourcery.ai/Reference/Default-Rules/refactorings/remove-redundant-if/))
</issue_to_address>