diff --git a/telegram_export/utils.py b/telegram_export/utils.py
index 0212e51..851c888 100644
--- a/telegram_export/utils.py
+++ b/telegram_export/utils.py
@@ -128,6 +128,9 @@ def get_extension(mime):
     Returns the most common extension for the given mimetype, or '.bin' if
     none can be found to indicate that it contains arbitrary binary data.
     """
+    if not mime:
+        mime = ''
+    
     return (
         COMMON_MIME_TO_EXTENSION.get(mime)
         or mimetypes.guess_extension(mime)