Attachments aren't supported in any type of campaign, including ones originating from the API. Instead of a traditional attachment, you can include a hyperlink to the file hosted in your Media Library or on an external server.
Tips before you begin:
1. Verify that the file name does not contain spaces or special characters
2. Verify the file size is not more than 4.0 MB
3. Upload the file to your Media Library (Assets > Media Library > click Upload)
- Open your draft campaign and navigate to step 2: Design.
- Select the content block to edit
- Highlight text you'd like hyperlinked to the file
- Click Insert Link
- Click Browse (the three dots on the site of the website URL)
- Click Documents tab
- Select your file from the library
- Click Ok to insert the link
For linking to PDFs hosted in Delivra, please see this article.