FILTER RESULTS USING OR

[Drew Pierce (Surf)]

Quite often there a list of file types I am looking for. Currently it seems I need to filter out the types of files I am not interested in to be left with a list of files I am interested in. 

Is there a syntax that works as OR in the filter?

.avi,.mp4,.mkv

doesnt work

(.avi,.mp4,.mkv)

doesnt work 

.avi or .mp4 or .mkv

doesnt work

-.pdf -.epub -.mobi -.op -.txt -.az -.b -.c -.d -.h -.l -.mp3 -.fl -.wav

does work 

I dont understand why it works to subtract off a list of extensions but does not work to specify a list of acceptable file types.