Custom File Names

Hey, would you accept a pull request to allow filename processing? In short a way to configure the file name to be used for the storage strategies?

Specifically, I need a way to put the modelName, fieldName, model's ID, & a time stamp into the file name. The first 3 are for organization & the timestamp is for cache busting.

something like:
`/public/attachments/:modelName/:fieldName/:id-:updated_at-:guid.ext`

Start by locating the storage strategy implementation and how filenames are currently generated. Trace the attachment configuration and identify how model name, field name, ID, timestamp, and GUID could be supplied. Done means a configurable pattern produces the requested path and filename, with tests covering token expansion.

Written by the indexing model from the issue text.