Update allowed_file function in fileuploads.rst

Update allowed_file function to accept lowercase and uppercase file extensions

docs/patterns/fileuploads.rst

@@ -47,7 +47,7 @@ the file and redirects the user to the URL for the uploaded file::
 
     def allowed_file(filename):
         return '.' in filename and \
-               filename.rsplit('.', 1)[1] in ALLOWED_EXTENSIONS
+               filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS
 
     @app.route('/', methods=['GET', 'POST'])
     def upload_file():