filebackend: Added supported for retrieving file metadata/headers

* This can be useful for carrying over metadata when copying files around
* Also fixed a bug in sanitizeHdrs() for Swift (broken content-disposition)

Change-Id: I4534e9acac2b306086797b3677f85c05b98e39fc
(cherry picked from commit ac14c1c8a65d154f7a9bc47d1b51f6db67188f86)