[FIX] website: improve performance for attachment creation

website introduces two new stored function fields, which depend on the attachment data
The thing is, these two fields are pertinent for website attachments only
Therefore, we avoid to read the datas field when the attachment is not a website attachment(when not needed), as this is the most costly field to read
[FIX] survey: Was browsing a browse record, useless and traceback... https://github.com/odoo/odoo/issues/2383