[postprocessor/embedthumbnail] embed mp4 too (fixes #5840)

youtube_dl/postprocessor/embedthumbnail.py

@@ -49,7 +49,7 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
             os.remove(encodeFilename(filename))
             os.rename(encodeFilename(temp_filename), encodeFilename(filename))
 
-        elif info['ext'] == 'm4a':
+        elif info['ext'] in ['m4a', 'mp4']:
             if not check_executable('AtomicParsley', ['-v']):
                 raise EmbedThumbnailPPError('AtomicParsley was not found. Please install.')
 
@@ -82,6 +82,6 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
                 os.remove(encodeFilename(filename))
                 os.rename(encodeFilename(temp_filename), encodeFilename(filename))
         else:
-            raise EmbedThumbnailPPError('Only mp3 and m4a are supported for thumbnail embedding for now.')
+            raise EmbedThumbnailPPError('Only mp3 and m4a/mp4 are supported for thumbnail embedding for now.')
 
         return [], info