Add option for limiting file types in search

[Clorith]

Adding a way to limit which file types are searched could beneficial in certain scenarios.

See original feature request at https://wordpress.org/support/topic/search-only-php-files/

[primathon]

+1 to this. Even just being able to add a modifier to the search query (similar to how Google does it) like "filetype:php" or "filetype:js" would be amazing. Often the initial point of entry I have in debugging a third-party plugin issue is something like an element class name or ID, and trying to find where that's generated in the PHP will almost always show many results I don't care about in the plugin CSS files. Being able to limit the search files to a particular extension would be very useful.