[FIX] base: reference attach, not self in _compute_raw()

Current behavior:
call to _file_read() within "for attach in self:" loop references self
rather than attach

Expected behavior:
execute _file_read() individually for every record in self (via "for
attach in self:" loop)

This is not an issue in standard where _file_read is an api.model
method but in case of overwrite (e.g. issue reported at
odoo/odoo#60016 has implemented an AWS integration) it makes sense

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>