Display a BoM.
On BoM lines, there is an attachment feature that creates a mrp.document,
which is basically a container for an attachment.
This commit fixes the view which displayed 'no attachment' when there were some
and vice versa.
It also fixes the computation of the number of attachment: the displayed view is
on mrp.document, but the computation is done on ir.attachment.
As a result if there are other unrelated attachments (such as digital files)
they would be counted but not displayed.
opw 1923441
closesodoo/odoo#30208
Nans Lefebvre
ba65d0e259