Something very similar has been discussed in this thread:
including some pieces that would create strange results when treated automatically.
As a first approach: if you have the files in separate folders, then you could run the track numbering wizard to include the totals and then check the total number of files as stored in the field TRACK.
You could filter or check these numbers and then apply the action to fill the relieasetype (which is a user-defined field anyway and will not be picked up by most players - MEDIATYPE would come closest, I think)
The length, though, will not be included