func (file *GridFile) SetContentType(ctype string) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	file.doc.ContentType = ctype
	file.m.Unlock()
}
func (file *GridFile) SetContentType(ctype string) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	file.doc.ContentType = ctype
	file.m.Unlock()
}
// 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.