if a MantisTouch url has been set in the mantis config (g_mantistouch_url) then mantis will always redirect all requests from a mobile to the MantisTouch instance. This is fine most of the time however when trying to view an image attached on a bug you end up redirected to the MantisTouch home page.
I verified that when g_mantistouch_url is blank it works correctly. Should be a minor update to core.php (around line 112) to detect what the person is trying to look at and not perform the redirection if it is an attachment.
This brings up a separate issue which is that you are required to re-authenticate with mantis when you do try to view this image but perhaps this is more of a feature request to provide cross auth.