Search for folders that contain filetypes For example, how do you search for all folders that contain ".mp3" files (on OS X)?
I don't think it's possible using Finder, but you can easily find these folders via Terminal

find . -name '*.mp3' | sed 's|/[^/]*$||' | sort -u


Some explanations:


find lists all files (and their paths) ending in mp3 in the current folder and everything "beneath" it
sed strips away the last part of each line produced by find which does not contain a / (which practically results in the file name getting removed)
The result so far is a list of folders containing mp3 files. If a folder contains more than one such file, it will be listed several times so we use sort -u (unique) to remove duplicates while sorting.