Add support for attachment deletion

Checklist

• I have used the search function to see if someone else has already submitted the same feature request.
• I will describe the problem with as much detail as possible.
• This issue only contains a request for one single feature, not multiple (related) features.

App version

6.202

Problem you are trying to solve

I'm doing a lot of mail triaging with K-9 while committing, and K-9 really shines at that task.
One stumbling stone for me is attachment management.
It often happens to me that I'd like to archive an email, but only after having deleted some or all of its (large) attachments.
Unless I'm missing something, attachment deletion is not currently supported in K-9.
So when I'm in that situation I just keep the email around in my mailbox, waiting for when I'll use another mail client (webmail, mutt, etc.) just for attachment deletion before archival.
Would it be possible to add an attachment deletion feature?

Suggested solution

A new action can be added to attachment boxes, either as a button near the download one or as a long-press action.

Screenshots / Drawings / Technical details

No response

Start by locating the attachment boxes and the existing download action in the Android app. Determine how an attachment deletion action should work and confirm the behavior needed for deleting attachments before archiving; done means users can remove selected attachments from a message.