zulip/static/templates/lightbox_overlay.hbs

<div id="lightbox_overlay" class="overlay new-style" data-overlay="lightbox">
    <div class="image-info-wrapper">
        <div class="image-description">
            <div class="title"></div>
            <div class="user"></div>
        </div>
        <div class="exit" aria-label="{{t 'Close' }}"><span aria-hidden="true">x</span></div>
        <div class="image-actions">
            <div class="lightbox-canvas-trigger">
                <div class="title">{{t "Pan & zoom" }}</div>
                <div class="status" data-disabled="{{t 'Disabled' }}" data-enabled="{{t 'Enabled' }}"></div>
            </div>
            <a class="button small open" rel="noopener noreferrer" target="_blank">{{t "Open" }}</a>
            <a class="button small download" download>{{t "Download" }}</a>
        </div>
        <div class="clear-float"></div>
    </div>

    <div class="image-preview overlay-content no-select">
        <div class="zoom-element"></div>
    </div>
    <div class="player-container"></div>
    <div class="center">
        <div class="arrow no-select" data-direction="prev">&lt;</div>
        <div class="image-list"></div>
        <div class="arrow no-select" data-direction="next">&gt;</div>
    </div>
</div>