[FIX] base: avoid indexing attachment on copy

When sending a mass mailing with a pdf attachment, the ir.attachment
is copied in the mail.composer for each email. Spending time and
resources in the indexing of the pdf. On copy that value is already in
the vals. This is especially true when attachment_indexation is
installed, processing the pdf documents can be costly with pdfminer.

It could also be done by overwriting the copu from ir_attachment but it
could happen a misuse of mass create of ir.attachment with the same file
has the same issue

closes odoo/odoo#99856

Signed-off-by: Raphael Collet <rco@odoo.com>