[downloader:ytdl] fix file extensions when merging into mkv

gallery_dl/downloader/ytdl.py

@@ -70,6 +70,10 @@ class YoutubeDLDownloader(DownloaderBase):
         if "url" in info_dict:
             text.nameext_from_url(info_dict["url"], pathfmt.kwdict)
 
+        formats = info_dict.get("requested_formats")
+        if formats and not compatible_formats(formats):
+            info_dict["ext"] = "mkv"
+
         if self.outtmpl:
             self.ytdl.params["outtmpl"] = self.outtmpl
             pathfmt.filename = filename = self.ytdl.prepare_filename(info_dict)
@@ -105,4 +109,15 @@ class YoutubeDLDownloader(DownloaderBase):
         return True
 
 
+def compatible_formats(formats):
+    video_ext = formats[0].get("ext")
+    audio_ext = formats[1].get("ext")
+
+    if video_ext == "webm" and audio_ext == "webm":
+        return True
+
+    exts = ("mp3", "mp4", "m4a", "m4p", "m4b", "m4r", "m4v", "ismv", "isma")
+    return video_ext in exts and audio_ext in exts
+
+
 __downloader__ = YoutubeDLDownloader