diff --git a/anytask/courses/views.py b/anytask/courses/views.py
index 0af0b209..1717a5bc 100644
--- a/anytask/courses/views.py
+++ b/anytask/courses/views.py
@@ -642,7 +642,7 @@ def course_settings(request, course_id):
                 issue.set_teacher(teacher=teacher)
 
     if 'rb_extensions[]' in request.POST:
-        course.filename_extensions = request.POST.getlist('rb_extensions[]')
+        course.filename_extensions.set(request.POST.getlist('rb_extensions[]'))
     else:
         course.filename_extensions.clear()