[FIX] [base, account_edi_facturx]: Fix pdf export

There is an issue when exporting pdf using edi documents created before
the PDF/A commit. With the subtype now included, the system would try
to use the subtype given by the ir.attachment which would not be formated
as expected by the pdf file format.

The attachment may get neutered by the ORM, so we may have to force
the mimetype when embedding it.

This fix in two parts will allow to force a subtype when adding an
attachment into a pdf, as well as parse the subtype of ir.attachment
to give them the right format.

xxx/xxx should become /xxx#2Fxxx

opw-2714040

closes odoo/odoo#81709

Signed-off-by: Laurent Smet <las@odoo.com>