diff --git a/youtube_dl/postprocessor/embedthumbnail.py b/youtube_dl/postprocessor/embedthumbnail.py
index 56be914b8..e2002ab0b 100644
--- a/youtube_dl/postprocessor/embedthumbnail.py
+++ b/youtube_dl/postprocessor/embedthumbnail.py
@@ -41,6 +41,28 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
                 'Skipping embedding the thumbnail because the file is missing.')
             return [], info
 
+        # Check for mislabeled webp file
+        with open(encodeFilename(thumbnail_filename), "rb") as f:
+            b = f.read(16)
+        if b'\x57\x45\x42\x50' in b:  # Binary for WEBP
+            [thumbnail_filename_path, thumbnail_filename_extension] = os.path.splitext(thumbnail_filename)
+            if not thumbnail_filename_extension == ".webp":
+                webp_thumbnail_filename = thumbnail_filename_path + ".webp"
+                os.rename(encodeFilename(thumbnail_filename), encodeFilename(webp_thumbnail_filename))
+                thumbnail_filename = webp_thumbnail_filename
+
+        # If not a jpg or png thumbnail, convert it to jpg using ffmpeg
+        if not os.path.splitext(thumbnail_filename)[1].lower() in ['.jpg', '.png']:
+            jpg_thumbnail_filename = os.path.splitext(thumbnail_filename)[0] + ".jpg"
+            jpg_thumbnail_filename = os.path.join(os.path.dirname(jpg_thumbnail_filename), os.path.basename(jpg_thumbnail_filename).replace('%', '_'))  # ffmpeg interprets % as image sequence
+
+            self._downloader.to_screen('[ffmpeg] Converting thumbnail "%s" to JPEG' % thumbnail_filename)
+
+            self.run_ffmpeg(thumbnail_filename, jpg_thumbnail_filename, ['-bsf:v', 'mjpeg2jpeg'])
+
+            os.remove(encodeFilename(thumbnail_filename))
+            thumbnail_filename = jpg_thumbnail_filename
+
         if info['ext'] == 'mp3':
             options = [
                 '-c', 'copy', '-map', '0', '-map', '1',
diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index d1eca3760..c73f5e0ca 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -4173,6 +4173,7 @@ def mimetype2ext(mt):
         # Per RFC 3003, audio/mpeg can be .mp1, .mp2 or .mp3. Here use .mp3 as
         # it's the most popular one
         'audio/mpeg': 'mp3',
+        'audio/x-wav': 'wav',
     }.get(mt)
     if ext is not None:
         return ext