email does not recognize attachments

We have users who send emails with attachments in order to create an issue, but although the issues is created, no attachment is recognized. The sender's email program is MAC OS X Mail. We tried sending as an in-line attachment and also by clicking on the paper clip and answering Yes to 'Send Windows Friendly Attachment'. I've tried reading the issues on receiving inline attachments but I couldn't find where/how to fix this. Since we have no real control over the way our clients send us attachments, it would be nice to either fix this or figure out what to warn them about.