feat: add contentType parameter to Attachment and EmailEndpoint.attach method

Author: lucasra1

src/main/java/co/lettermint/endpoints/EmailEndpoint.java

@@ -151,7 +151,20 @@ public EmailEndpoint attach(String filename, String content) {
      * @param contentId Content-ID for inline attachments (e.g., for embedding images)
      */
     public EmailEndpoint attach(String filename, String content, String contentId) {
-        this.attachments.add(new Attachment(filename, content, contentId));
+        this.attachments.add(new Attachment(filename, content, contentId, null));
+        return this;
+    }
+
+    /**
+     * Attach an inline file to the email and specify the {@code Content-Type}
+     *
+     * @param filename    The filename to use for the attachment
+     * @param content     Base64-encoded file content
+     * @param contentId   Content-ID for inline attachments (e.g., for embedding images)
+     * @param contentType MIME type of the attachment (e.g., "image/png")
+     */
+    public EmailEndpoint attach(String filename, String content, String contentId, String contentType) {
+        this.attachments.add(new Attachment(filename, content, contentId, contentType));
         return this;
     }
 

src/main/java/co/lettermint/models/Attachment.java

@@ -18,14 +18,18 @@ public class Attachment {
     @JsonProperty("content_id")
     private final String contentId;
 
+    @JsonProperty("content_type")
+    private final String contentType;
+
     public Attachment(String filename, String content) {
-        this(filename, content, null);
+        this(filename, content, null, null);
     }
 
-    public Attachment(String filename, String content, String contentId) {
+    public Attachment(String filename, String content, String contentId, String contentType) {
         this.filename = filename;
         this.content = content;
         this.contentId = contentId;
+        this.contentType = contentType;
     }
 
     public String getFilename() {
@@ -39,4 +43,8 @@ public String getContent() {
     public String getContentId() {
         return contentId;
     }
+
+    public String getContentType() {
+        return contentType;
+    }
 }

src/test/java/co/lettermint/EmailEndpointTest.java

@@ -81,6 +81,7 @@ void testEmailWithAllOptions() throws Exception {
                 .headers(headers)
                 .attach("document.pdf", "base64content")
                 .attach("logo.png", "base64logo", "logo-cid")
+                .attach("invite.ics", "base64icalFile", null, "text/calendar; method=REQUEST")
                 .route("route-slug-123")
                 .metadata(metadata)
                 .tag("welcome", "onboarding")
@@ -103,6 +104,9 @@ void testEmailWithAllOptions() throws Exception {
         assertTrue(body.contains("\"text\":\"Hello World\""));
         assertTrue(body.contains("\"route\":\"route-slug-123\""));
         assertTrue(body.contains("\"tags\":[\"welcome\",\"onboarding\"]"));
+        assertTrue(body.contains("{\"filename\":\"document.pdf\",\"content\":\"base64content\"}"));
+        assertTrue(body.contains("{\"filename\":\"logo.png\",\"content\":\"base64logo\",\"content_id\":\"logo-cid\"}"));
+        assertTrue(body.contains("{\"filename\":\"invite.ics\",\"content\":\"base64icalFile\",\"content_type\":\"text/calendar; method=REQUEST\"}"));
     }
 
     @Test