Add commas between allowed file extensions
You are definitely allowed to separate file types by comma in the admin are



For user area, find the ID of the form (you can find this in the firebug) then 

function YourTheme_form_alter(&amp;$form, &amp;$form_state, $form_id){
      if($form_id == "TheFormID"){
            kpr($form);  
      }
}


In this way you can find the form elements. get the Key of image field in the form array, if it is image then write

function YourTheme_form_alter(&amp;$form, &amp;$form_state, $form_id){
      if($form_id == "TheFormID"){
            $form['image']['#description'] = t('File types are comma separated')
      }
}


This is the general idea but a few modification might be needed