[FIX] mail: dl inline image with original filename
[FIX] mail: dl inline image with original filename

In 599bd509 for inline image in a mail the `Content-Id` of the
attachment was used as the file name to avoir an issue of several inline
images with the same name only displaying the first one.

This solved the issue but with it the file name when downloading the
image were lost as well as the extension which can be an issue.

This changeset refactor the code so the behavior stay the same but the
original file names are kept.

opw-725305

hip-odoo/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo