Gotta love that iPhone.
Rumor has it, that if you "Share" it the Apple system will automatically convert it to a normal .jpg before sending it on its' way.
Since it is not a broadly accepted or for that matter broadly used format, you'll likely need to resort to a converter.
I see there are a few out there including some batch style on-line converters.
Apple’s iPhone and iPad now take photos in the HEIF image format, and these photos have the .HEIC file extension. Windows doesn’t natively support HEIC files, but there’s a way to view them anyway—or convert them to standard JPEGs.