support filepaths to .contentType()

I noticed the readme makes it seem as though file paths will please the .contentType() method, but when used with a path like "here/is/my/file.json", the directory separator `/` is being mistaken for a content-type's `/`, which is interpreted as "I don't need you to lookup the content-type". Instead of looking for a `/`, I changed it look for a `;`. Hope this isn't a terrible solution!