How can I get the ActivePresentation file format from PowerPoint Interop
...for the enum Microsoft.Office.Interop.PowerPoint.PpSaveAsFileType?
You actually can't get the file format, unfortunately. The best that can be done is to read the extention, like in this thread: How to obtain PowerPoint File Format Programmatically.
One other way is to use DSOFile to read the format.
Once the presentation is loaded it has no file format, only the internal memory structure. So from that POV it doesn't make sense to have the fileformat of the active presentation. That being said, using the extension is a horrible way to determine file format...though it's better than nothing. The best way is to "sniff" the file, as mentioned in the second answer here