Adding a delete button for webform attachment fields
There is an example in the examples module.

There is a module with in that called form_example. If you check the 9th example you would get what you are looking. It has an example of adding and removing fields dynamically and the code is well documented. 

change value '#value' =&gt; t('Remove'), to '#value' =&gt; t('Remove' . $i),

and in function you could get that as given below

function form_remove_attachment($i) {
    $clicked = explode(' ', $form_state['clicked_button']['#value']);
   //$clicked[1] would have the value of $i

    $form_state['rebuild'] = TRUE;
}