Note total number of items imported, and number of items that are not convertable. From the non-convertable tab, identify what applications may be missing.
Some file types (like movies) cannot be converted. Some file types like .DAT may in fact be Text files (and can be assigned to TXT). If you do install additional software, you need to do a re-check for the file type to be recognized. Place-holders for unconvertable files can be created using the pass-thru feature.
Detailed tweaking of file types can be done from the Admin / Configure / Documents Dialog.