This is an old version of this answer!
Return to the current answerinsert this code to functions.php of your theme.
add_filter('attachment_fields_to_edit', 'medium_attachment_fields_to_edit', 0, 2);
function medium_attachment_fields_to_edit($fields, $post){
if (substr($post->post_mime_type,0,5) == 'image'){
$fields['url']['html'] = str_replace("<button type='button' class='button urlfile'", "<button type='button' class='button urlfile' title='".esc_attr(array_shift( wp_get_attachment_image_src($post->ID, 'medium', false) ))."'>Medium File URL</button>\n<button type='button' class='button urlfile'", $fields['url']['html']);
}
return $fields;
}
Oleg Butuzov | 06/24/10 at 5:22pm





