Apparently version 2.9.15 broke PPTX indexing. There’s a PowerPoint file that can import just fine using 2.9.14 but can’t be imported with a “file type unrecognized” error in 2.9.15.
Thanks for the bug report, we’ll check this. This should affect only the search index, previewing should still work as expected.
BTW:
Which version of PowerPoint do you use?
I’m having the same issue. The PwerPoint version is 15.38.
This will be fixed in the next maintenance release.