Add notice about stripped, but not stored attachments

[cloph]

currently attachments are silently dropped, there is no indication when an attachment was removed.

Person sens mail with png and zip attachment, png is allowed, zip is not → attachment-page will only show the png, but nowhere it mentions the zip attachment.

Ideally, it would list the attachments and their status in the mail itself, and not only in the attachment-system.

something like

Attachments <url to attachment-system>:
foobar.png - stored (expires: <somedate>)
barfoo.zip - rejected (filetype not allowed)

would be my idea of it...

rejection reasons could be one of filetype not allowed, size limit exceeded, virus found, or something like that