The docx, xlsx and pptx file types extend the Office Open XML file format: https://en.wikipedia.org/wiki/Office_Open_XML_file_formats We look for: - one entry named '[Content_Types].xml' or '_rels/.rels', - one entry indicating specific type of file. MS Office, OpenOffice and LibreOffice may put the parts in different order, so the check should not rely on it.
function findNextZipHeaderIndex(arr) {
			var startAt = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 0;
			return arr.findIndex(function (el, i, arr) {
				return i >= startAt && arr[i] === 0x50 && arr[i + 1] === 0x4B && arr[i + 2] === 0x3 && arr[i + 3] === 0x4;
			});
		}