The 'My Documents’ folder is for your private files, and other users cannot access that folder.
When you attach a file to a conversation from your ‘My Documents’ folder, other users cannot see the path to the file or reveal it in the Storage. Other users can only preview or download the file.
When you attach a file to a conversation from your computer, the file is stored in the My Documents folder and it increases Storage usage, but the file remains hidden. Other users cannot see the path to the file or reveal it in the Storage, they can only preview or download the file.