Occurs before the user removes a single file from the list of files to upload (when clicks on button ).

Parameters:

realName - (string) the real name of the file (as it's displayed in the control)
serverName - (string) the file name that the server returns after uploading or null if the file wasn't uploaded to the server

myForm.attachEvent("onBeforeFileRemove", function(realName, serverName){
    if (!confirm("Are you sure you want to delete "+realName+"?")) return false;
    return true;
})

Returning false will cancel the 'remove' operation.