Commit 74fd75555669cfe0a53b6cbc50a425e6f9f093d1

A null mimetype is not an enclosure (more likely than not means there was an error)
  
285285 $enclosure->mimetype=$this->mimetype;
286286
287287 if(! isset($this->filename)){
288 $notEnclosureMimeTypes = array('text/html','application/xhtml+xml');
288 $notEnclosureMimeTypes = array(null,'text/html','application/xhtml+xml');
289289 $mimetype = strtolower($this->mimetype);
290290 $semicolon = strpos($mimetype,';');
291291 if($semicolon){