MacBook Air screenshots saving as .jp, not .jpg

Does this also happen if you manually copy a properly-named file into the screenshots destination?

AFAIK, there is already something wonky happening here, as Mac usually uses either PNG or HEIC files.

Has somebody been using Terminal here, maybe? To check:

defaults read com.apple.screencapture type

Here is a lengthy related Tahoe thread Did Tahoe block changing the default scre… - Apple Community

Apparently the Terminal command (and using the third party utility Onyx) still work under Tahoe. There's some debate but it may simply be because the person wanted the file type as PDF. You could use either method to change the type to jpg

That's odd because that is not a file type for anything. Are you sure you aren't simply not seeing the "g" at the end?

I see you are using Tahoe. I only see mention of changing from HDR to SDR using the Screenshot options (under command+shift+5 Take a screenshot on Mac - Apple Support). Even with earlier OS versions the only type Apple actually offered was PNG. It was possible (using Terminal commands or third party utilities) to change this to JPG. Did you do this when using another OS version? If you did, I wonder if Tahoe somehow corrupted this in the transition.

I hesitate to tell you how to do this on your Mac using Terminal commands that have been used in earlier system versions because while I do have my Sequoia Mac set to save Screenshots as JPG (I probably used a utility, which you should only use with caution), I don't know if this would mess up Tahoe if it absolutely insists upon using one of the formats it wants to use.