Another one bug with gio.
#/usr/bin/env ruby require 'gio2' Gio::ContentType.guess('filename.mp3')
$ ruby test.rb
/usr/lib/ruby/site_ruby/2.1.0/gio2/loader.rb:145:in `invoke': no implicit conversion from nil to integer (TypeError)from /usr/lib/ruby/site_ruby/2.1.0/gio2/loader.rb:145:in `block in load_function_info_content_type'
from test.rb:5:in `<main>'
Downloaded from here https://github.com/ruby-gnome2/ruby-gnome2
Thanks for your report!
I've fixed it!