This is most likely because your PC does not recognise the HEIC format. In 2017, Apple began using the High Efficiency Image Container (HEIC) format in iOS 11 and macOS High Sierra. This format allows for smaller file sizes than JPG or PNG.
If you would prefer that your iPhone camera saves images in JPG format, you can go to Settings/Camera/Formats and set the ‘Most Compatible’ option. Your camera will now save all images a JPGs – but be aware that your images will now be larger in filesize.
Windows 10 supports HEIC files, however earlier versions of Windows may not. If you wish to view your HEIC photos on Windows and your PC does not recognise them, we recommend converting them to JPG using this handy online tool: https://heictojpg.com/