Discovered this problem in IE7 (probably exists in IE6 too) where the FileUpload control textbox allows invalid user input that prevents the form from posting back to the server. This is not a problem in IE8 and FF where the FileUpload control textbox renders properly in readonly mode. You can work around this problem by adding javascript  to the 'onkeypress' and 'onkeydown' event handler to prevent users from typing any potentially invalid string. The javascript function (which uses cross-browser javascript keycode detection) below will block all typing except tabs - this should allow users to still tab over controls.

function blockAllExceptTab(e) {
   
    if (!e) var e = window.event
    if (e.keyCode) code = e.keyCode;
    else if (e.which) code = e.which;
   
    var result = (code == 9);
    return result;
}

Add this code to your page OnInit event handler:

this.myFileUpload.Attributes.Add("onkeypress", "return blockAllExceptTab(event);");
this.myFileUpload.Attributes.Add("onkeydown", "return blockAllExceptTab(event);");

Hope this helps.