if you're using flash.filesystem.FileReference - then it is run in black box - except of filename, size and creation data (and few other properties available after some operation succeeded). This is part of security features in Flash runtime (described in header section):
This for example implies that user can download a content to local machine - but that content cannot be loaded back into Flash runtime. For this you would need either Air runtime flash.filesystem.File:
(so you would created Air runtime based application: desktop or mobile but not web, even as standalone project) or you would need one of 3rd party tools that add file access/file information features to standard Flash runtime applications converted to standalone native applications.