I have written an own module for my reports in Odoo 8.

addons/my_reports/my_reports_report.xml

<report
    id="my_report_saleorder"
    string="My Offers"
    model="sale.order"
    report_type="qweb-pdf"
    name="my_reports.my_reports_saleorder"
    file="my_reports.my_reports_saleorder"
    attachment_use="True"
    attachment="'Angebot-ausgehend_'
 + (object.name or '').replace('/','_')
 + '_'
 + (object.partner_id.name or '').replace('/', '_').replace(' ', '-')
 + (object.state == 'draft' and '_draft' or '')
 + '.pdf'"
/>

addons/my_reports/views/my_reports_saleorder.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>
<template id="my_reports_saleorder_document">
        ... many report related stuff ...
</template>
<template id="my_reports_saleorder">
    <t t-call="report.html_container">
        <t t-foreach="doc_ids" t-as="doc_id">
            <t t-raw="translate_doc(doc_id, doc_model, 'partner_id.lang', 'my_reports.my_reports_saleorder_document')"/>
        </t>
    </t>
</template>
</data>
</openerp>

It is attached automatically to the print menu dropbox of the sale_order form.

If I print my report it will be automatically attached to the form. But it is not attached as 1 document. It will be attached multiple documents. One document for every page in the report. (I have installed 'Document Management System 2.1'.)

My invoice report works correctly.

Has any body an idea?