[thumbnailsconvertor] Treat `jpeg` as `jpg`

yt_dlp/postprocessor/embedthumbnail.py

@@ -92,7 +92,7 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
         # format, there will be some additional data loss.
         # PNG, on the other hand, is lossless.
         thumbnail_ext = os.path.splitext(thumbnail_filename)[1][1:]
-        if thumbnail_ext not in ('jpg', 'png'):
+        if thumbnail_ext not in ('jpg', 'jpeg', 'png'):
             thumbnail_filename = convertor.convert_thumbnail(thumbnail_filename, 'png')
             thumbnail_ext = 'png'
 

yt_dlp/postprocessor/ffmpeg.py

@@ -896,6 +896,8 @@ class FFmpegThumbnailsConvertorPP(FFmpegPostProcessor):
             _, thumbnail_ext = os.path.splitext(original_thumbnail)
             if thumbnail_ext:
                 thumbnail_ext = thumbnail_ext[1:].lower()
+            if thumbnail_ext == 'jpeg':
+                thumbnail_ext = 'jpg'
             if thumbnail_ext == self.format:
                 self.to_screen('Thumbnail "%s" is already in the requested format' % original_thumbnail)
                 continue