Derive content-type from filename extension for reliable rendering

app.py

@@ -77,7 +77,7 @@ async def upload_file(
     file_id = uuid.uuid4().hex
     content = await file.read()
     filename = file.filename or "file"
-    content_type = file.content_type or mimetypes.guess_type(filename)[0] or "application/octet-stream"
+    content_type = mimetypes.guess_type(filename)[0] or file.content_type or "application/octet-stream"
     safe_name = filename.replace(" ", "_")
 
     file_key = f"uploads/{file_id}/{safe_name}"
@@ -156,7 +156,8 @@ def serve_file(file_id: str):
     r2_key = meta["r2_key"]
     obj = s3.get_object(Bucket=R2_BUCKET, Key=r2_key)
     body = obj["Body"]
-    content_type = meta.get("content_type", "application/octet-stream")
+    filename = meta.get("filename", "")
+    content_type = mimetypes.guess_type(filename)[0] or meta.get("content_type") or "application/octet-stream"
 
     def stream():
         while True: