Adding an attachment and setting the MIME type to text/xml (either autosuggested or manually) will result in an error #400.
Created attachment 8806 [details] Example for that Bug with Gentoo's index.xml Adding an attachment with "auto-detect" switched on
Resulted Error-Message: Attachment #8806 [details] to Bug #16536 Created Status: 400 Bad request (malformed multipart POST) Note: Bugzilla automatically detected the content type text/xml for this attachment. If this is incorrect, correct the value by editing the attachment.
it's not just xml files ... it seems like any kind of post results in the error ...
*** This bug has been marked as a duplicate of 16172 ***