Images and videos do show in post format, but those are quite small and typically I open such in another tab so I can see them, but rather than showing such, I just get the save-as dialogue with a .file extension.
Oddly if I hit save as, it adds the correct extension.
@Suiseiseki that's a pretty interesting finding. Everything works completely fine for me though, at least when I visit https://freesoftwareextremist.com. How did you determine the file extension? When I right-click on the video and press "save video as", I get this filename: c6bfd170ccb28a2372fa9b69ebd3da3f14edd2bdb806947357dae25336407a32.mp4 so everything appears to be fine.
@Suiseiseki apparently I've been using media_post somewhat incorrectly. The file_name argument can be omitted, but until now I put "file" in it. I'm just gonna remove the argument.