So I found that PowerPoint slide shows that had been added into MCMS were being retrieved as just plain PowerPoint files. What this really meant was that the file’s mime-type and extension had been changed from ‘PPS’ to ‘PPT’.
This was a pain – it’s just not as pretty as having the file open as a presentation.
It turns out the culprit for this was IIS – it didn’t have a mime-type defined for ‘.pps’. I fixed this using the advice from Microsoft’s support pages – PowerPoint Show (*.pps) file copies as a PowerPoint (.ppt) file from an IIS Web server.
What the article doesn’t mention, though, is that for MCMS, this change needs applied not to the website, but to the ‘NR/rdonlyres’ directory within the MCMS site.