[Alansmith]

For future folks looking for answers:

It seems there is an issue with using Dropbox as the intermediate to move the file to Instagram. One cannot (for some reason) upload to IG directly from the desktop. In the past, I have used the upload-> Other-> Dropbox from within the IG phone app, but this apparently no longer works (possibly just on Android, my platform). A little info here:
https://www.socmedsean.com/solved-posting-photos-to-instagram-from-your-desktop/
(at the bottom, in a quote)
So, the workaround is to put into dropbox, but then in the dropbox app download to the phone (to do this: in the app, click and hold the file, top right corner (three vertical dots) click and select ‘Save to device’. Select the ‘Movies’ folder. Then, in Instagram, upload as usual and you should see the item as an option.

-A