When I go onto Krita and save an artwork, it changes into a .kra file, and it won't allow me to upload onto other sites such as Imgur.
I tried changing it into a .png file, but it won't allow me to do it.

Any suggestions?

Try to choose png in the file-filter dropdown, or type '.png' to the end of your file. (or do both)

We are having this problem with some file browsers, but not others, making it very tricky to fix.