I want to filter every mp3 files in certain folder and its sub-folders. I typed *.mp3 in the search box but it is not working.
In the appearing dialog you can enter a regex, such as
*.mp3, in order to select all matching file names in the folder itself.
You can search/find.
Press Ctrl+F and enter the extension, like
.ogg and all the files with the extension
.ogg will be shown, depending on the directories you search in.
It's not quite what you asked for, but it's as close as I can think of.