social/mail_preview_base/static/src/xml/preview.xml

<?xml version="1.0" encoding="UTF-8" ?>
<templates xml:space="preserve">
    <t t-extend="DocumentViewer.Content">
        <t t-jquery=".o_viewer_img" t-operation="attributes">
            <attribute
                name="t-attf-src"
            >#{widget.activeAttachment.source_url}?unique=1&amp;signature=#{widget.activeAttachment.checksum}</attribute>
        </t>
        <t t-jquery=".o_viewer_pdf" t-operation="attributes">
            <attribute
                name="t-attf-src"
            >/web/static/lib/pdfjs/web/viewer.html?file=#{widget.activeAttachment.source_url}</attribute>
        </t>
        <t t-jquery=".o_viewer_video > source" t-operation="attributes">
            <attribute
                name="t-attf-src"
            >#{widget.activeAttachment.source_url}</attribute>
        </t>
        <t t-jquery=".o_viewer_text:first" t-operation="attributes">
            <attribute
                name="t-attf-src"
            >#{widget.activeAttachment.source_url}</attribute>
        </t>
    </t>

    <t t-extend="mail.Attachment">
        <t t-jquery="t[t-set='has_preview']" t-operation="after">
            <!--
                We have added integration to text, as it was already
                defined on the preview.
                TODO: Set the has_preview on a function...
            -->
            <t t-set="has_preview" t-value="has_preview || attachment.has_preview" />
        </t>
    </t>
</templates>