Try to register the givenFPreloadableFileinstance to handle the next call to TryTakeArchive for its FileName. Will fail if the instance has not been initialized or if another instance has already registered for the Filename. Return whether the instance is currently registered. Returns true if the instance was already registered. Registered files are referenced-counted, and the reference will not be dropped until (1) (TryTakeArchive or UnRegister is called) and (2) (PreloadBytes mode only) the archive returned from TryTakeArchive is deleted.