Rational
It is a common need for user to be able to share attachments with external people (using email).
Also these sharing may need to expire.
Proposal
A list (One2Many) of sharing secrets is added to the ir.attachment which can have an expiration date set.
The user can fill a comment to explain the sharing (ex: “Share with supplier XYZ”).
On creation link is generated like <database name>/ir/attachment/<id>/<secret>/<name>.
If an email address is set, an email is sent to the address with the link on creation. A button allow to resent the email. The email address is read-only to force user to delete and create new entry if the address must be changed.
When the web_shortener module is activated, it is used to shorten the URL instead of the long one.
In addition, we could also add an option to store as attachment a report that is generated and thus share it.