What is this file attach error?

Hi,

I'm the maintainer of the OG User Roles module: http://www.drupal.org/project/og_user_roles.

A user is getting this error on file attachments:

http://drupal.org/files/issues/og_user_role_errmsg.gif

The files attach and can be uploaded, but this error message pops up. I've no idea what it is.

Can someone please explain

a) why it pops up, and;
b) what should be done about it?

Thanks so much!