Attachments Getting Stuck when downloading in Apple Mail (as per previous thread 2018)

I am using Apple Mail with 2015 MacBook Pro and Mojave. Occasionally (e.g. Today) but not all the time, attachments don't open. There will be an image icon (such as image.#JPEG or what looks like a text edit doc but may be a QuickBooks doc) with small blue writing below that says "downloading . . ." but nothing seems to happen. Curiously, the Quickbooks doc that had the same blue message for a long time miraculously opened for me a few minutes ago. Maybe if I wait long enough the Photo images will download but it seems like it is taking a very long time.

NOTE: I reopened a thread from 2018 which discussed the same intermittent problem b/c Apple said it was so long ago I had to ask again. In that thread, someone said they had a workaround of sending to their Iphone and then the attachments downloaded and then they re sent to their e-mail IMap acct. I haven't tried this b/c of time and also b/c I don't typically do e-mail on my phone.