diff --git a/docs/use-cases/attachments.md b/docs/use-cases/attachments.md
index baeceec32..5eb67fe2e 100644
--- a/docs/use-cases/attachments.md
+++ b/docs/use-cases/attachments.md
@@ -16,7 +16,7 @@ const msg = {
       filename: 'some-attachment.txt',
       type: 'plain/text',
       disposition: 'attachment',
-      content_id: 'mytext'
+      contentId: 'mytext'
     },
   ],
 };
@@ -43,7 +43,7 @@ fs.readFile(('Document.pdf'), (err, data) => {
           filename: 'some-attachment.pdf',
           type: 'application/pdf',
           disposition: 'attachment',
-          content_id: 'mytext',
+          contentId: 'mytext',
         },
       ],
     };
@@ -72,7 +72,7 @@ request(fileURl, { encoding: null }, (err, res, body) => {
           filename: 'some-attachment.pdf',
           type: 'application/pdf',
           disposition: 'attachment',
-          content_id: 'mytext',
+          contentId: 'mytext',
         },
       ],
     };
diff --git a/packages/helpers/classes/attachment.d.ts b/packages/helpers/classes/attachment.d.ts
index fba0f8be2..f81b022c1 100644
--- a/packages/helpers/classes/attachment.d.ts
+++ b/packages/helpers/classes/attachment.d.ts
@@ -30,4 +30,6 @@ export default class Attachment implements AttachmentData {
   setDisposition(disposition: string): void;
   setContentId(contentId: string): void;
   toJSON(): AttachmentJSON;
+  static create(data: AttachmentData | Attachment): Attachment;
+  static create(data: AttachmentData[] | Attachment[]): Attachment[];
 }
diff --git a/packages/helpers/classes/attachment.js b/packages/helpers/classes/attachment.js
index 46beed47b..a50cf0f2c 100644
--- a/packages/helpers/classes/attachment.js
+++ b/packages/helpers/classes/attachment.js
@@ -181,6 +181,19 @@ class Attachment {
     //Return
     return toSnakeCase(json);
   }
+
+  static create(data) {
+    if(Array.isArray(data)) {
+      return data.filter(Boolean).map(item => this.create(item));
+    }
+    //Already instance of Attachment class?
+    if (data instanceof Attachment) {
+      return data;
+    }
+
+    //Create instance
+    return new Attachment(data);
+  }
 }
 
 //Export class
diff --git a/packages/helpers/classes/mail.js b/packages/helpers/classes/mail.js
index b7dce14a2..221a5aa28 100644
--- a/packages/helpers/classes/mail.js
+++ b/packages/helpers/classes/mail.js
@@ -5,6 +5,7 @@
  */
 const EmailAddress = require('./email-address');
 const Personalization = require('./personalization');
+const Attachment = require('./attachment');
 const toCamelCase = require('../helpers/to-camel-case');
 const toSnakeCase = require('../helpers/to-snake-case');
 const deepClone = require('../helpers/deep-clone');
@@ -388,7 +389,7 @@ class Mail {
       if (!attachments.every(attachment => !attachment.disposition || typeof attachment.disposition === 'string')) {
         throw new Error('Expected the attachment\'s `disposition` field to be a string');
       }
-      this.attachments = attachments;
+      this.attachments = Attachment.create(attachments);
     }
   }
 
diff --git a/packages/helpers/classes/mail.spec.js b/packages/helpers/classes/mail.spec.js
index 41aec6be9..83695553b 100644
--- a/packages/helpers/classes/mail.spec.js
+++ b/packages/helpers/classes/mail.spec.js
@@ -185,6 +185,23 @@ describe('Mail', function() {
           }],
         })).to.throw('filename');
       });
+
+      it('properly handles attachment contentId', () => {
+        const contentId = 'test-content-id';
+        const mail = new Mail({
+          to: 'recipient@example.org',
+          attachments: [{
+            disposition: 'inline',
+            content: 'test-content',
+            filename: 'name-that-file',
+            type: 'file-type',
+            contentId,
+          }],
+        });
+        const mainJSON = mail.toJSON();
+        const firstAttachment = mainJSON.attachments[0];
+        expect(firstAttachment.content_id).to.equal(contentId);
+      });
     });
   });