Attachments which come from Email or user on downloading giving error.

I have using maian ticket support system. The attachments coming from imap email or from user end through their account on downloading give the error that either the file appears to be damaged, corrupted or is to large, but actually image file is 100 KB. When I go through cpanel all the attachment files are already present & view able.
so by the support admin panel it was not view able  or downloadable.
Please provide solution to it.