Error: No Preview available (Uploading PPTX)

i get the error no preview available.the preview must be first created in powerpoint?

 

Hello,
I have the same phenomenon with an Excel file. A PDF behaves as expected.
It;s on a public server, both files (excel and a PDF) in same folder.
I had a link here, but using the link the display differs.

Any clue?

Leo

A pdf from the site was too big to upload. I made a screenshot instead 

Google? I would be interested to embed Google docs and Sheets as well. Do you have that?
So far I suppose this concerns a Office files thing...

PDF's work fine. I had a graphical one, a scanned document  of 15 pages, half typed text, half maps an photocopies from old journals, weighing 3.67 MB, but no problems displaying it. pure text-pdf's also fine. 
The Excel was made for the test, a proof of concept, and is just a few kB a new Excel 2016 file having one text string, two numbers and a formula. I atrached it