[ThumbnailsConvertor] Support `webp`

Closes #2226

README.md

@@ -951,7 +951,7 @@ You can also fork the project on github and run your fork's [build workflow](.gi
                                      (currently supported: srt|vtt|ass|lrc)
                                      (Alias: --convert-subtitles)
     --convert-thumbnails FORMAT      Convert the thumbnails to another format
-                                     (currently supported: jpg|png)
+                                     (currently supported: jpg|png|webp)
     --split-chapters                 Split video into multiple files based on
                                      internal chapters. The "chapter:" prefix
                                      can be used with "--paths" and "--output"

yt_dlp/postprocessor/ffmpeg.py

@@ -1055,7 +1055,7 @@ class FFmpegSplitChaptersPP(FFmpegPostProcessor):
 
 
 class FFmpegThumbnailsConvertorPP(FFmpegPostProcessor):
-    SUPPORTED_EXTS = ('jpg', 'png')
+    SUPPORTED_EXTS = ('jpg', 'png', 'webp')
 
     def __init__(self, downloader=None, format=None):
         super(FFmpegThumbnailsConvertorPP, self).__init__(downloader)