Do action 'save_post' for attachments in 'wp_insert_attachment()'

[dnaber-de]

I'm currently change my plugins to use meta boxes instead of the filter 'attachment_fields_to_edit' to add fields to the attachment edit screen. Adding meta boxes works as expected like for all other post types.

They usual way to evaluate meta boxes is a callback on 'save_post' for all other post-types. At the moment there are two actions for the attachment post type: 'add_attachment' and 'edit_attachment'. I think it would be consistent to do the action 'save_post' also in 'wp_insert_attachment()'. Or at least provide an action like 'save_attachment'.

Related: #21391 - Move attachment editing (media.php) to use standard post type UI

adds save_post and edit_post actions to wp_insert_attachment

#21963