XForms/Multiple Uploads

Motivation
You would like the user to be able to attach multiple files to a form.

Method
We will use the  around the upload control:

User interface notes

 * 1) The delete trigger will only be visible after the second upload is shown.
 * 2) This interface shows the file name and size in bytes.  The mime-type could also be shown
 * 3) You can add any number of files
 * 4) You can delete any row by selecting the "Delete" button
 * 5) You can re-add a file using the red X

The files will be uploaded to the server in an async process.

Tested on Orbeon 4.3