check filename file upload

I want to check the extension of a filename in the fileld " file field " (name = file) before the submit button is clicked on.
I made the following javascript:

function CheckExtension(file)
{
  var extension = file.substr( (file.lastIndexOf('.') +1) );
  if(extension != "ly" && extension != "xml" && extension != "mxl")
  {
       alert("Verkeerde bestandextensie");
       return false;
   }
   return true;
}

But it does not work. I think because file is not the filename given in the field.

Greetings Theo

Hi Theo,

There are a couple of possible solutions in this StackOverFlow thread. You can also add an 'accep' attribute in the element Extra Params box that might help.

Bob