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;

	}