CSS from a sub directory of a custom mixed library directory is served as JS

Hi,

when using a custom library containing both scripts and stylesheets (see an example of configuration below)
    
     'paths' => array(
         '/custom/mixed/library/directory' => 'javascripts,stylesheets',
      ),
    
a request for a css file contained in a sub directory (ie `css/requested-file.css`) results in serving it with the content type : `application/javascript`.   
This comes from that the validation process of file extensions is not constrained to stylesheets (in that case) and javascripts are checked first.    

The provided fix is a little bit elaborated than simply hard-coding types, but it should be more flexible.