Just use jquery to check the value:
$(document).ready(function () {
$('#<%= YourFileUploadControl.ClientID %>').change( function(){
if ( $('#<%= YourFileUploadControl.ClientID %>').val() <> '' ){
$('#<%= YourButtonControl.ClientID %>').removeAttr('disabled');
}
else {
$('#<%= YourButtonControl.ClientID %>').attr('disabled', 'disabled');
}
});
});