muk_web/muk_web_preview_attachment/static/src/xml/sidebar_preview.xml

<?xml version="1.0" encoding="UTF-8"?>

<!--     
	Copyright (C) 2017 MuK IT GmbH
	
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU Affero General Public License as
    published by the Free Software Foundation, either version 3 of the
    License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Affero General Public License for more details.

    You should have received a copy of the GNU Affero General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>. 
-->

<templates>
    <t t-extend="Sidebar">
    	<t t-jquery="a:has(.o_sidebar_delete_attachment)" t-operation="replace">
    		<a t-att-title="item.title or None" t-att-data-section="section.name" t-att-data-index="item_index" t-att-href="item.url">
                <span class="o_sidebar_label"><t t-raw="item.label"/></span>
                <span t-if="section.name == 'files' and widget.options.editable and !item.callback" class="fa fa-trash-o o_sidebar_delete_attachment" t-att-data-id="item.id" title="Delete this attachment"/>
            </a>
    	</t>
        <t t-jquery="span.o_sidebar_delete_attachment" t-operation="before">
            <span t-if="section.name == 'files' and !item.callback and item.url" class="fa fa-file-o o_sidebar_preview_attachment" t-att-data-id="item.id" title="Preview this attachment"/>
        </t>
    </t>
</templates>