[imp] Attachment sizes and linked attachments

[Chuck Hagenbuch]

Quoting Jan Schneider <jan at horde.org>:

> If you can't cover your external service directly with an existing VFS
> driver, the easiest way would be to create a small custom VFS driver
> that wraps your service. I can also see us supporting different
> storage methods than VFS, but I first have to be convinced that such a
> storage method can't be covered by a VFS driver.

I agree; since we already have a driver model here, I'd be very  
reluctant to add another abstraction layer on top of that for linked  
attachments.

-chuck