I am having the exact same problem. In my case, it is on an UPDATE page so the server file name is no good for updating the recordset because the only way I can reliably make an update page work with DFP is to have the user copy the filename from the Select Box to the field which actually updates the db. They copy and paste a filename with JPG but DFP uploads a file with jpg.
Why is DFP changing this when most webservers are going to recognize image.jpg and image.JPG as two separate files?