Make filefield_file_download() work with Module Grants module

( If you don't want to include this: the issue may at least serve as documentation for others in the future. )

Module Grants basically provides a better content access mechanism in cases where node_access() doesn't cut it.
One example -my case- is a proper content moderation system, using the Revisioning module.
At this moment, there are nodes on my site which
- are unpublished and pending moderation, i.e. moderators can see them
- have filefields that trigger a call to hook_file_download.

Moderators cannot download the files, because a node_access() call for the node will return FALSE. (Access to the node page itself is determined by module_grants_node_access(), which returns TRUE in this case.)

The only way I saw to fix this, is to patch filefield_file_download().