Issue number rewriting shouldn't rewrite file attachments

[GoogleCodeExporter]

Issue number rewriting shouldn't rewrite file attachments. If there is a file 
attachment named "issue1234.patch" we shouldn't rewrite that as 
"issue4567.patch" to match GitHub. Instead, we should keep it as-is since 
otherwise the links will be invalid.

See:
https://github.com/firebug/firebug/issues/5234#issuecomment-103495972.

Which actually corresponds to:
http://storage.googleapis.com/google-code-attachments/fbug/issue-5096/comment-2/
issue5096.patch
http://storage.googleapis.com/google-code-attachments/fbug/issue-5096/comment-4/
Issue5096.patch

Original issue reported on code.google.com by chrsm...@google.com on 18 Jun 2015 at 4:51