Don't calculate mimetype if file is None

redditbot.py

@@ -207,7 +207,7 @@ async def main():
                         j = j[0]
                         k = k[0]
                 attributes = []
-                mimetype = await _get_file_mimetype(k)
+                mimetype = (await _get_file_mimetype(k)) if k else ''
                 if mimetype.startswith('video/'):
                     try:
                         data = await _get_video_data(k)