[FIX] mail: send same attachment multiple times

In the Discuss app, using Chrome or IE11, try to send the same
attachment to different channels. The first attachment is correctly
detected but not the next ones.

The problem is due to a browser inconsistency, on firefox selecting the
same file in an `<input type=file>` triggers the `change` event. This is
not the case on Chrome/IE, selecting the same file doesn't triggers an
`onchange`.

opw-2006647

closes odoo/odoo#34076

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>