mkv is better extension for the video, as we are using mkv envelope.

src/process_pipeline.c

@@ -620,17 +620,17 @@ format_movie_name(char *capture_fname)
 
 	if (g_get_user_special_dir(G_USER_DIRECTORY_VIDEOS) != NULL) {
 		sprintf(capture_fname,
-			"%s/VID%s.mp4",
+			"%s/VID%s.mkv",
 			g_get_user_special_dir(G_USER_DIRECTORY_VIDEOS),
 			timestamp);
 	} else if (getenv("XDG_VIDOES_DIR") != NULL) {
 		sprintf(capture_fname,
-			"%s/VID%s.mp4",
+			"%s/VID%s.mkv",
 			getenv("XDG_VIDEOS_DIR"),
 			timestamp);
 	} else {
 		sprintf(capture_fname,
-			"%s/Videos/VID%s.mp4",
+			"%s/Videos/VID%s.mkv",
 			getenv("HOME"),
 			timestamp);
 	}