fix: use named var in f-string

My Python 3.11.2 explodes otherwise.

upload.py

@@ -17,13 +17,13 @@ APP.config["SECRET_KEY"] = os.urandom(24)
 
 class ImageUploadForm(FlaskForm):
     """Image upload form."""
-
+    allowed = ", ".join(ALLOWED_FILES)
     image = FileField(
         "image:",
         validators=[
             FileAllowed(
                 ALLOWED_FILES,
-                f"Images only, please use any of the following file extensions: {(", ").join(ALLOWED_FILES)}",
+                f"Images only, please use any of the following file extensions: {allowed}",
             )
         ],
     )