Mail preview tab: fixed bug in mimetype lookup for attachments with a wrong mimetype.

(cherry picked from commit a72a868028d3fd16672ed374554ffc80e604f8d0)

Conflicts:
	docs/HISTORY.txt
Merge pull request #522 from 4teamwork/lg-backport-mimetypes-fix

Backport `Mail preview tab: fixed bug in mimetype lookup...` to 3.4-stable