[FIX] base: add get_extension helper

Since PR #90855, we add extension if mimetypes doesn't match the type.
Since guess_type don't know some extension, we prefer considere all string
of less of 8 char as an extension before to fallback on the mimetypes lib.

With this commit, a filename filename.scss will be considered with an
extension .scss by our own helper instead of a fallback on .bin or .a as
returned by guess_extension of mimetype.

closes odoo/odoo#91581

Signed-off-by: Julien Castiaux <juc@odoo.com>