Fix bug in processing of content types

owl/post.py

@@ -84,8 +84,8 @@ def validate_attachment(form, field):
     if not ctype.is_allowed():
         raise wtforms.ValidationError(f'Attachment type {mime_type} is not allowed in this course')
 
-    if extension not in ctype.allowed_exts:
-        extension = ctype.allowed_exts[0]
+    if extension not in ctype.extensions:
+        extension = ctype.extensions[0]
 
     field.owl_mime_type = mime_type
     field.owl_extension = extension