Please allow attaching textual files with any extension, any filetype

[geekley]

Is your feature request related to a problem? Please describe.
It's a bad idea to just whitelist some file extensions that can be attached.

Describe the solution you'd like
Any file detected as textual should be allowed as an attachment.
If the file is binary, then you might restrict to supported types if needed.

Describe alternatives you've considered
If you need a MIME type that you can't detect (e.g. from the OS) for some reason, allow it to be manually entered. Or just use generic "plaintext". But please don't just force us to rename e.g. a .jsonc to .json, or a file in some less known programming language to .txt or whatever.

Additional context
I wish you could just drag the files as well.

Not sure if this is a duplicate of #382