mail-addons/mail_attachment_popup/static/src/xml/mail_attachment_popup.xml

<?xml version="1.0" encoding="UTF-8"?>
<template>
    <t t-extend="mail.thread.message.attachments">
        <t t-jquery="t[t-if='attachment.file_type_icon === 'webimage''] a[t-att-href='attachment.url']" t-operation="replace">
            <span class="m-dotted" t-attf-onclick="$('#ImageModal{{ attachment.id }}').arcticmodal()">
                <img t-att-src="widget.attachments_resize_image(attachment.id, [100,80])"></img>
                <div class='oe_name'><t t-raw='attachment.name' /></div>
            </span>
            <div class="g-hidden">
                <div class="box-modal" t-attf-id="ImageModal{{ attachment.id }}">
                    <div class="box-modal_close arcticmodal-close">X</div>
                    <img t-att-src="widget.attachments_resize_image(attachment.id, [100,80])"></img>
                    <div class='oe_name'><t t-raw='attachment.name' /></div>
                    <div class='oe_download_original_img'><a t-att-href='attachment.url' target="_blank">Download</a></div>
                </div>
            </div>
        </t>
    </t>
</template>