Allow for arbitrary file types in attachments

This is basically repeating/reopening https://www.amplenote.com/suggested_features/8 (as I wasn't sure if it made sense to comment there after it was marked done).

It is also this public feature suggestion, I think: https://amplenote.featureupvote.com/suggestions/561131/attach-any-file-type-size-can-be-limited

In particular I'd like to be able to attach text files and variants of them (incl. particularly source code, Markdown files, HTML and MHTML), but I don't know of a reason off-hand to limit things to any particular content type at all (though I see the advantage to having *handlers* for specific types that can do extra things, of course).

I'm curious if there are reasons why this isn't already there or if you think supporting arbitrary file types is a bad idea?