onBeforeFileAdd

Occurs before the user adds a file to the upload queue.

Parameters:

• realName - (string) the real name of the file (as it's displayed in the control)
• size - (int) the size of the file (not supported for html4 mode)

myForm.attachEvent("onBeforeFileAdd ",function(realName, size){
    // any custom logic here
    return true;
});

• Returning false will cancel the 'add' operation.
• For example - this event can be used to filter files by the extension.