Hehehehehe...yeah, I'll see if I can get that in the next version.
In the mean time, you might be able to do a search filter in Windows Explorer by extension, and then just temporarily move out the files that you don't want into another folder (but that probably won't work if you're using subfolders). Ah, you could back up the entire folder (make a copy), delete all the non-ROM files (should be easy by filtering or sorting on extension in Windows Explorer), run the import, and then replace the folder with your backup copy. A big pain, yeah.