feat: Add inline attachment

src/main/java/com/resend/services/emails/model/Attachment.java

@@ -15,10 +15,14 @@ public class Attachment {
     @JsonProperty("path")
     private final String path;
 
+    @JsonProperty("content_id")
+    private final String contentId;
+
     private Attachment(Builder builder) {
         this.fileName = builder.fileName;
         this.content = builder.content;
         this.path = builder.path;
+        this.contentId = builder.contentId;
     }
 
     /**
@@ -45,6 +49,14 @@ public String getPath() {
         return path;
     }
 
+    /**
+     * Get the content ID for inline attachments used in HTML content with cid: references.
+     * @return The content ID for inline attachments.
+     */
+    public String getContentId() {
+        return contentId;
+    }
+
     /**
      * Create a new Attachment builder.
      * @return A new Builder instance.
@@ -60,6 +72,7 @@ public static class Builder {
         private String fileName;
         private String content;
         private String path;
+        private String contentId;
 
         /**
          * Set the filename of the attachment.
@@ -91,6 +104,16 @@ public Builder path(String path) {
             return this;
         }
 
+        /**
+         * Set the content ID for inline attachments used in HTML content with cid: references.
+         * @param contentId The content ID for inline attachments.
+         * @return The Builder instance.
+         */
+        public Builder contentId(String contentId) {
+            this.contentId = contentId;
+            return this;
+        }
+
         /**
          * Build an Attachment instance.
          * @return The built Attachment.

src/test/java/com/resend/services/emails/EmailsTest.java

@@ -10,9 +10,7 @@
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
 
-import java.util.Collections;
 import java.util.List;
-import java.util.Map;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;

src/test/java/com/resend/services/util/EmailsUtil.java

@@ -17,6 +17,7 @@ public static Attachment createAttachment() {
         return Attachment.builder()
                 .fileName("invoice.pdf")
                 .content("invoice.pdf")
+                .contentId("my-image")
                 .build();
     }