code: func (file *GridFile) SetContentType(ctype string) {
  	file.assertMode(gfsWriting)
  	file.m.Lock()
  	file.doc.ContentType = ctype
  	file.m.Unlock()
  }
docs: // ContentType changes the optional file content type.  An empty string may be
  // used to unset it.
  //
  // It is a runtime error to call this function when the file is not open
  // for writing.