wagtail/wagtail/admin/templates/wagtailadmin/icons/collapse-down.svg

<svg id="icon-collapse-down" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16"><!--! Custom icon --><rect width="2" height="12" x="14" y="1.5" rx="1" transform="rotate(90 14 1.5)"/><path d="M8 13.625a.928.928 0 0 1-.674-.264L2.64 8.674a.903.903 0 0 1 0-1.319.903.903 0 0 1 1.318 0L8 11.37l4.014-4.014a.903.903 0 0 1 1.318 0c.38.352.38.967 0 1.319L8.645 13.36a.909.909 0 0 1-.645.264Z"/></svg>