Fix issue attachment handling (#24202)

Close #24195

Some of the changes are taken from my another fix
https://github.com/go-gitea/gitea/pull/20147/commits/f07b0de997125c9b79cc5af27966a7cdd1803a4d
in #20147 (although that PR was discarded ....)


The bug is:

1. The old code doesn't handle `removedfile` event correctly
2. The old code doesn't provide attachments for type=CommentTypeReview


This PR doesn't intend to refactor the "upload" code to a perfect state
(to avoid making the review difficult), so some legacy styles are kept.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

models/issues/issue.go

@@ -269,7 +269,7 @@ func (issue *Issue) LoadPullRequest(ctx context.Context) (err error) {
 }
 
 func (issue *Issue) loadComments(ctx context.Context) (err error) {
-	return issue.loadCommentsByType(ctx, CommentTypeUnknown)
+	return issue.loadCommentsByType(ctx, CommentTypeUndefined)
 }
 
 // LoadDiscussComments loads discuss comments