muk_web/muk_web_preview/demo/preview_demo.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data noupdate="1">
	
	<record id="pdf_attachment_demo" model="ir.attachment">	    
		<field name="name">sample.pdf</field> 
		<field name="datas_fname">sample.pdf</field>
	    <field name="datas" type="base64" file="muk_web_preview/demo/data/sample.pdf"/>
	</record>
	
	</data>
</odoo>