This works for me (but please make a backup of the files before!):
wp-admin/includes/media.php (about ll. 605)
replace
$edit_post = sanitize_post($post, 'edit');
$file = wp_get_attachment_url($post->ID);
$link = get_attachment_link($post->ID);
with
$edit_post = sanitize_post($post, 'edit');
$file = wp_get_attachment_url($post->ID);
$medium = ereg_replace(".jpg","-medium.jpg",$file);
$link = get_attachment_link($post->ID);
wp-admin/includes/media.php (about ll. 635)
replace
<button type='button' class='button url-$post->ID' value=''>" . __('None') . "</button>
<button type='button' class='button url-$post->ID' value='" . attribute_escape($file) . "'>" . __('File URL') . "</button>
<button type='button' class='button url-$post->ID' value='" . attribute_escape($link) . "'>" . __('Post URL') . "</button>
with
<button type='button' class='button url-$post->ID' value=''>" . __('None') . "</button>
<button type='button' class='button url-$post->ID' value='" . attribute_escape($file) . "'>" . __('File URL') . "</button>
<button type='button' class='button url-$post->ID' value='" . attribute_escape($medium) . "'>Medium</button>
<button type='button' class='button url-$post->ID' value='" . attribute_escape($link) . "'>" . __('Post URL') . "</button>
wp-includes/media.php (about ll. 224)
replace
$suffix = "{$dst_w}x{$dst_h}";
with
if($max_w == get_option('thumbnail_size_w') || $max_h == get_option('thumbnail_size_h')) { $suffix = "thumb"; }
elseif($max_w == get_option('medium_size_w') || $max_h == get_option('medium_size_h')) { $suffix = "medium"; }
else { $suffix = "{$dst_w}x{$dst_h}"; }
Now you get two thumbnails: a small one (filename-thumb.jpg) and a medium one (filename-medium.jpg). You can change the size in the WordPress config.
It would be great if anyone could pack that into a plugin.
Viele Grüße und sorry for my bäd English!