[development] hook_fileapi (file products and download permission)

[Nedjo Rogers]

Dave,

So far as I can tell, the problem you're addressing indeed exists as you 
describe it: there is no opening in the file api to enable nuanced file 
access control. And it should be addressed--there's no sense in us having to 
override/replace our file api just to control access (as was done, e.g., in 
the ecommerce file.module).

Your suggested approach for handling access looks basically sound to me--but 
it would be more useful to hear from more experienced fileapi-ers. Walkah, 
killes, dopry, Chris Johnson, etc.?

Likley we'd want to keep it as close as possible to existing _api and 
_access hooks. So, e.g.,

* @param $op
*   The type of event ('upload', 'download', 'update', 'delete')

might be better as

* @param $op
*   The type of event ('insert', 'view', 'update', 'delete')

Are you thinking of doing this as a contrib module for 4.7? Part of 
ecommerce, or a generic file handling module?