Mail stuck "Downloading" attachment
Problem: I would open an email a day or two after receiving it, and any/all attachments will appear, but show "downloading", with an empty status bar that would never complete. I had this issue and found multiple threads with no resolution.
In Sierra 10.13.6, Mail version 11.5, I solved it by doing the following:
Click on Mail, Preferences, Accounts. Click the individual email account(s) from the list on the left. To the right, click the "Account information tab". At the bottom, there is a dropdown labeled "Download Attachments" Set this to "All". Mine was defaulted to "Recent", which apparently is less than 24 hours...
Wait for a few minutes for the system to sync/download - depending on how many attachments are in limbo, it might take a while.