If a blob is already file-backed, and especially if it's in its own file, then we can do a simple file copy - or even do a file move into the idb blob file directory).

Complications:
* Which quota system are we using for these blobs?
* Can we update the referenced blob to be referencing the IDB blob?

Goals:
* Avoid duplicate data storage on disk
* Avoid needing to copy by bytes, and use file system operation.