i might be a bit late, but you simply have to encode it in b64 ;)    

I am using this code to upload image to ir.attachment for product.template and it works for me. Hope it will help. Odoo will automatically add records for image_medium and image_small.

   $imageBase64 = base64_encode(file_get_contents('path_to_your_file/image.png'));
   $models->execute_kw($db, $uid, $password,
    'ir.attachment',
    'create', // Function name
        array( // Values-array
            array( // First record
                'res_model'=>'product.template',
                'name' => 'image',
                'res_field' => 'image',
                'res_id'=> 1234,
                'res_name' => 'product_name',
                'type' => 'binary',
                'datas' => $imageBase64
            )
        )
    );

The only question i had is to create an attachement i need to request as the following:

$param = array(array(

            'res_model'=>'account.invoice',

            'res_id'=>95,

            'name' => 'myattachement.pdf',

            'datas' => '????'

        ));

  $id = $models->execute_kw($db,$uid,$password,

        'ir.attachment',

        'create',

        $param);

My question is How to pass the data ?