FEAT: Select multiple attachments with a keyboard shortcut

[MDM808]

Could you add in a future version the ability to add multiple attachments at the same time by selecting them individually with a keyboard shortcut?

[andrinoff]

Hey, again! 👋

I have thought about it, while implementing multi-file attachments in #227. I didn't seem to figure out how to make it at the same time easy-to-use and efficient.

There are a few problems:

1. How to make it so that it is obvious to the average user?
2. When the attachments are selected, how to deselect them in a way that you don't deselect the wrong files, or all of them at once?

If you can describe the structure in detail, or submit a PR with changes, I'd be happy to check it out or review

[MDM808]

It could be with some key combination to select-deselect a single file, and another combination to select-deselect all at the same time.