While setting the media content in the Composer Online template, the different attachment-related collections are suggested for selection. 

Attached Content Document 

This collection pulls up all of the related Images that you uploaded as Files. This is done in the Files related list. Salesforce switches to the Files framework to allow you more flexibility with your uploaded information - here you can upload such image formats as jpg., png., etc.

Attachment

This collection pulls up images from the AscendixRE Image Viewer component.  

Note, Attachment, Google Doc and File and Note and Attachment 

This collection pulls up all the files stored in the Notes & Attachments or Files tabs, including:

• images;
• files (.pdf, .docs, etc.)
• images from AscendixRE Image Viewer
• notes from Activities.

Note and Attachment

This collection pulls up the images uploaded via AscendixRE Image Viewer and notes from Activities.