Implementation as cache plugin

[LecrisUT]

There can potentially be some hiccups if in the task lists you have two lookups of the same attachment with more than the ttl of the socket in between. In those cases the path variable still points to the temp file location, but in the meantime the database might have been closed, deleting the file.

Alternatively, maybe the attachment lookup can be coupled to a cache plugin so that we can expire it when the database is closed. I am not sure if we can have more than one cache plugin, but there seem to be plugins like aws_ec2, which I would suspect only target specific variables.