Including PowerPoint files as book format

I do many presentations. I use Calibre to store and organize my presentation materials by event. Among the files I need to store powerpoint files (.pptx) as well. Although .pptx file is not officially supported by Calibre, I can change the file extension option in the file open dialog to include "All files *.*" to select pptx.

Currently I switch to Mac. There is no such filetype option on Mac. I cannot select pptx files as they are grey in color.

Is there a way to include powerpoint files (or file with any file type) to calibre?

Thanks