File Input Field handles the upload by itself (in principal, it maps to a <INPUT TYPE="FILE"> tag) - once a user selects a file using the Browse button, the <Value> element (of type File) "contains" the file.
Manipulating a File type structures is limited by the browser security model. There's actually only one thing you can do with it - pass it in its entirety to a server side process (such as Basic/Service) - once it is passed to the server side, the Content can be accessed.
There are a few example which you may check out to understand more:
For best results, use the Firefox browser..