They are raw media files with MXF metadata.
It is not a Quicktime file. It is more analogous to *.DV files. Incidently, QT knows by default how to handle *.DV files. QT 7 might be able to handle DIFs.
Theoretically it can be used by any application that can interpret MXF file structure AND has the codec [DV25/DV50/100] to decompress the file.
Avid handles MXF natively. You can drag and drop the file in to a bin, all media and metadata is sorted for you automatically.