The FileReference class provides a means to upload and download files
between a user's computer and a server.
An operating-system dialog box prompts the user to select a file to upload
or a location for download.
Each FileReference object refers to a single file on the user's disk
and has properties that contain information about the file's size, type,
name, creation date, modification date, and creator type (Macintosh only).
A file's extension is the part of the name following (and not including) the final dot (".").
If there is no dot in the filename, the extension is null.
You should use the extension property to determine a file's type;
do not use the creator or type properties.
You should consider the creator and type properties
to be considered deprecated. They apply to older versions of Mac OS.
Implementation public function get extension():String