Henri I have already found out that previewing docx and xlsx files in files.gallery is not possible.
That would be correct, because Files Gallery can only preview text-based documents (txt, javascript etc) or formats that are supported directly in the browser, like PDF. This could only work for example by forwarding the file to Google docs for example. Something I might look into in a future release.
Henri When trying to download an xlsx or docx file, I only get an html file with the docx extension as the result of the download.
Do you have a link I can check? It works fine in our demo gallery:
https://demo.files.gallery/?files#filter=docx
Send link to karl @ photo.gallery if it is private.