Got the following via email from a user:

We have a problem with MantisTouch concerning the download of attachments. It's not possible for us to download attachments while on the MantisTouch Site. I manually changed the code in core.php to not redirect the file_download page in case of a mobile browser. That makes the download work if i change the useragent in firefox to a mobile browser. The Problem is that the issue still persists on a real mobile devices. I tried to debug it, and the problem seems to be, that in mobile browser, a request to the login page is sent first. During the redirection that follows, the "return" parameter gets lost, so the wanted document can't be loaded. Do you have any idea what could cause that?