Currently it is impossible to edit the attachment after they have been uploaded. I find that it is a pity since an attachment is nothing but a special post. Moreover the right line is there in the
post.php file and it requires a non-existent file:
Replacing this file by
edit-form-advanced.php apparently does the trick. It'd be great if some of the developers could give some advice about that. Even better, if someone has written the file
edit-attachment-form.php, i'd like to have a look at it.