diff --git a/youtube_dl/postprocessor/embedthumbnail.py b/youtube_dl/postprocessor/embedthumbnail.py
index 90002460c..c5c3cbf24 100644
--- a/youtube_dl/postprocessor/embedthumbnail.py
+++ b/youtube_dl/postprocessor/embedthumbnail.py
@@ -97,33 +97,33 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
                 from mutagen.oggopus   import OggOpus
                 from mutagen.flac      import Picture, FLAC
                 from base64            import b64encode
+            except ImportError:
+                raise EmbedThumbnailPPError('mutagen was not found. Please install.')
 
-                # to prevent the behaviour of in-place modification of Mutagen
                 shutil.copyfile(filename, temp_filename)
-
                 aufile = {'opus': OggOpus, 'flac': FLAC, 'ogg': OggVorbis} \
                          [info['ext']](temp_filename)
                 
                 covart = Picture()
                 covart.data = open(thumbnail_filename, 'rb').read()
-                covart.type = 3 #< use as front cover.
+                covart.type = 3 # Cover (front)
 
+                # Since, OGGOpus and OGGVorbis doesn't natively support
+                # (coverart / thumbnail)s, it's wrapped in if..else
                 if info['ext'] == 'flac':
                     aufile.add_picture(covart)
                 else:
-                    # VorbisComments don't allow raw-bytes so it's encoded
-                    # in base64 and converted to a string.
                     aufile['metadata_block_picture'] = \
                         b64encode(covart.write()).decode('ascii')
                 
+                # Save changes to temporary file, it'd be overlapped as the
+                # original one.
                 aufile.save()
 
                 if not self._already_have_thumbnail:
                     os.remove(encodeFilename(thumbnail_filename))
                 os.remove(encodeFilename(filename))
                 os.rename(encodeFilename(temp_filename), encodeFilename(filename))
-            except ImportError:
-                raise EmbedThumbnailPPError('mutagen was not found. Please install.')
         else:
             raise EmbedThumbnailPPError('Only mp3, m4a/mp4, ogg, opus and flac are supported for thumbnail embedding for now.')