Looks like we have two versions of jQuery loading. Hence the "(...).once is not a function" errors in the console. jquery.once is definitely loaded but only for the first version of jQuery.
How do I track down where the second one is coming from... (rhetorical question)?
~~ FIRST UPDATE
Having established this is a theme problem, what could be missing that would prevent selected files from uploading with the rest of the comment?
~~ ORIGINAL POST
This has me completely stumped.
I have attached a file field (not media, for reasons) to a comment type.
I attach a file to a new comment, and save.
The comment is saved. The file is not. And there are no errors. None.
Nothing in the Drupal log at all.
It's not a permissions error, because no error message and because I tried with a node type and that worked fine. I've tried both private and public, same thing.
Examining the DB shows that there is nothing in the field table, or in the file table.
The files I'm using are test files I use all the time, they aren't too big just standard PDF files. (And the file type is correct - but like I said, there are no error messages anyway.)
We are doing some fancy stuff with themes, but for the file field only standard Drupal formatting is being applied. (I checked thoroughly with Twig debug switched on.)