title: Embedding album cover in MP4 file using Mutagen
body: <p>I'd like to be able to add album cover into the file using mutagen, however when I add it as a file it returns with:</p>
  
  <blockquote>
  <pre><code>File "D:\Download\pandora\renamingMETAEFF.pyw", line 71, in &lt;module&gt;
      meta['covr'] = image
  File "C:\Users\AMD\AppData\Local\Programs\Python\Python35\lib\site-packages\mutagen\_file.py", line 67, in __setitem__
      self.tags[key] = value
  File "C:\Users\AMD\AppData\Local\Programs\Python\Python35\lib\site-packages\mutagen\mp4\__init__.py", line 357, in __setitem__
      self._render(key, value)
  File "C:\Users\AMD\AppData\Local\Programs\Python\Python35\lib\site-packages\mutagen\mp4\__init__.py", line 371, in _render
      return render_func(self, key, value)
  File "C:\Users\AMD\AppData\Local\Programs\Python\Python35\lib\site-packages\mutagen\mp4\__init__.py", line 732, in __render_cover 
      b"data", struct.pack("&gt;2I", imageformat, 0) + cover))
  
  TypeError: can't concat bytes to str
  </code></pre>
  </blockquote>
  
  <p>The relevant piece of code is this:</p>
  
  <pre><code>from mutagen.mp4 import MP4
  
  image = jpgname + '.jpg'
  meta['\xa9nam'] = song
  meta['\xa9ART'] = artist
  meta['\xa9alb'] = album
  meta = MP4(songPath)
  meta['covr'] = image
  meta.save()
  </code></pre>
  
  <p>The rest of the metadata works perfectly fine, however the image part completely breaks the whole code.</p>