Use filetype package to check file content for application/octet-stream content type header

Author: norkans7

Diff for: attachments.go

@@ -115,7 +115,7 @@ func FetchAndStoreAttachment(ctx context.Context, b Backend, channel Channel, at
 
 	// prioritize to use the response content type header if provided
 	contentTypeHeader := trace.Response.Header.Get("Content-Type")
-	if contentTypeHeader != "" {
+	if contentTypeHeader != "" && contentTypeHeader != "application/octet-stream" {
 		mimeType, _, _ = mime.ParseMediaType(contentTypeHeader)
 		if extension == "" {
 			extensions, err := mime.ExtensionsByType(mimeType)

Diff for: attachments_test.go

@@ -23,6 +23,9 @@ func TestFetchAndStoreAttachment(t *testing.T) {
 		"http://mock.com/media/hello2": {
 			httpx.NewMockResponse(200, map[string]string{"Content-Type": "image/jpeg"}, testJPG),
 		},
+		"http://mock.com/media/hello3": {
+			httpx.NewMockResponse(200, map[string]string{"Content-Type": "application/octet-stream"}, testJPG),
+		},
 		"http://mock.com/media/hello.mp3": {
 			httpx.NewMockResponse(502, nil, []byte(`My gateways!`)),
 		},
@@ -82,6 +85,12 @@ func TestFetchAndStoreAttachment(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, &courier.Attachment{ContentType: "unavailable", URL: "http://mock.com/media/hello.pdf"}, att)
 
+	att, err = courier.FetchAndStoreAttachment(ctx, mb, mockChannel, "http://mock.com/media/hello3", clog)
+	assert.NoError(t, err)
+	assert.Equal(t, "image/jpeg", att.ContentType)
+	assert.Equal(t, "https://backend.com/attachments/338ff339-5663-49ed-8ef6-384876655d1b.jpg", att.URL)
+	assert.Equal(t, 17301, att.Size)
+
 	// an actual error on our part should be returned as an error
 	mb.SetStorageError(errors.New("boom"))