Why are my photos showing up as videos on Windows and iCloud?

iPhones offer what are called "Live" Photos-- pictures like in Harry Potter where the photo isn't just an instant in time, but is a second or two of "life." Those pictures are viewable only on Photos (well, maybe on "The Daily Prophet" delivered by your owl.) When these Live pictures are transferred outside of Photos, they become a still image and a movie.

Unfortunately, you don't tell us what iOS you are using, so some things may have changed since yours. You can turn off "Live" Photos on your iPhone's camera with this button:

I have mine turned off, so there is a slash through the icon.

If you would like to keep your setting so that Live stays off the next time you use your camera, then you can tell the camera to preserve your settings by going to Settings>Camera>Preserve Settings>Live Photos and turning on the switch there. 

CB2367 wrote: … Seems that was turned on by default or got enabled through an update at some point, as I don’t recall enabling this myself.

It may be that "Live" is the default setting with a new phone--I have a 16, but I'm not sure how "Live" was set when I got it. Preserve settings' default is "off."