My best guess is that application/mspowerpoint is the old name for that Media Type. I can see some websites showing this Media Type as the one for PowerPoint, but none of them seem official enough.
I checked https://www.iana.org/assignments/media-types/media-types.xhtml (which I believe it can be trusted 
) and as you state, the correct Media Type for powerpoint seems to be application/vnd.ms-powerpoint.
And actually, application/mspowerpoint is nowhere to be seen on the iana website for media types.
Would you mind creating a ticket in the Akka HTTP repo?