Sending an email when a CPT is saved/updated (NOT when it’s created)

If you have registered that CPT and added uploads functionality, then you can write email functionality in that only. However, if it comes from any other plugin or theme, then either you will need to edit its code or create a plugin if sufficient hook is available.

If there is no custom hook available and you are going to use some generic WordPress hook e.g. save_post for your attachment, then make sure to check for your CPT’s post type before running email functionality, otherwise it will be applied for all the attachments.

e.g. you can check on save_post hook, if the post_type is attachment and the current screen is of your CPT, then send email.