1) Go to the page with the DMX Module
2) Mouse over the module options
3) Choose "Options"
4) Go to the "Service" tab
5) Click "File Types"
6) Find the file type(s) in question.
7) Click the "Edit" link for the file type(s)
8) If the "Default Action" is "View", change it to "Download"
Let me know if that helps.
Brian