Before, we would just throw 500 if a user passes an attachment that is
not an allowed type. This commit catches this error and throws a 422
instead since this should be considered a validation error.
(cherry picked from commit 872caa17c0a30d95f85ab75c068d606e07bd10b3)
Conflicts:
tests/integration/api_comment_attachment_test.go
tests/integration/api_issue_attachment_test.go
trivial context conflict because of 'allow setting the update date on issues and comments'
9cd0441cd3