Ralf Brown discusses the structure of .docx, .pptx, and .xlsx files, which are XML files inside a ZIP archive. He notes that the first member file is usually "[Content_Types].xml", but some files generated by LibreOffice may place this file later. He suggests possibly skipping the "[Content_Types].xml" test. Since MSOOXML lacks an uncompressed "mimetype" file like ePub or OpenDocument, he proposes scanning for a filename to distinguish between the three types.
documents/EFTA00502754.pdf
{}