In Odoo to upload and send a mail, this is the format. as an example

Obj = self.env['ir.attachment'].search([('res_name', '=', self.name)])
values = {
    'body': _('<p>Attached files : </p>'),
    'model': model.model,
    'message_type': 'comment',
    'no_auto_thread': False,
    'res_id': id_record,
    'attachment_ids': [(6, 0, obj and obj.ids)],
}
mail_id = request.env['mail.message'].sudo().create(values)

If you have already some set of file streams then do the following code to add attachments.

for file_name, file_content in files:
    part = MIMEBase('application', "octet-stream")
    part.set_payload( file_content )
    Encoders.encode_base64(part)
    part.add_header('Content-Disposition', 'attachment; filename="%s"'
                   % file_name)
    msg.attach(part)

If you are open a file directly then:

for f in files or []:
        with open(f, "rb") as fil:
            part = MIMEApplication(
                fil.read(),
                Name=basename(f)
            )
        # After the file is closed
        part['Content-Disposition'] = 'attachment; filename="%s"' % basename(f)
        msg.attach(part)

Above two examples are the codes from original send_mail() function. So if you know the attachment ids use the first one. :)