code: def maybe_convert_extension(ext)
        unless ext =~ /\A[a-zA-Z0-9_@-]+\z/
          # Don't look up strings unless they look like they could be a file
          # extension known to mime-types.
          #
          # There currently isn't any API public way to look up extensions
          # directly out of MIME::Types, but the type_for() method only strips
          # off after a period anyway.
          return ext
        end
  
        types = MIME::Types.type_for(ext)
        if types.empty?
          ext
        else
          types.first.content_type
        end
      end
docs: Given a MIME type or file extension, return either a MIME type or, if
   none is found, the input unchanged.
  
       >> maybe_convert_extension('json')
       => 'application/json'
  
       >> maybe_convert_extension('unknown')
       => 'unknown'
  
       >> maybe_convert_extension('application/xml')
       => 'application/xml'
  
   @param ext [String]
  
   @return [String]