support non-standard file extensions

issue jquery.min.map has a non-standard extension.

when a mime-type is not available the `MIME::Types.type_for(file)` returns an empty array the causes `type.first.content_type` to raise `undefined method `content_type' for nil:NilClass (NoMethodError)` exception
further control `!type.empty?` resolves the issue

You can say that I should use a public cdn for the jQuery library but it's only an example for a non-standard file extension
Merge pull request #14 from barvaz/master

support non-standard file extensions