diff --git a/asobi-downloader.py b/asobi-downloader.py
index 97ea6ce..46b81e8 100644
--- a/asobi-downloader.py
+++ b/asobi-downloader.py
@@ -301,6 +301,9 @@ def main() -> int:
     out_name = prompt_with_default(
         "Output filename (without path)", "output.mkv"
     )
+    if Path(out_name).suffix == "":
+        out_name = out_name + ".mkv"
+        print(f"No extension provided; using {out_name}")
     threads = int(prompt_with_default("Concurrent downloads", "16"))
     retries = int(prompt_with_default("Retries per file", "3"))
     meta_retries = int(prompt_with_default("Playlist/key retries", "3"))