Since this is the first time that HEIC is used as the photo format by Apple, and would be a new trend to replace JPEG in the near future, it's no wonder that HEIC won't be opened by most applications on Mac or Windows at present. Apple knows it as well and many software developers also have started to follow this tendency. So, there are some other feasible ways for you to tackle can't open HEIC photos on Mac/Windows problem well.
Solve HEIC won't open on Mac/Windows with your iPhone:
After your iPhone 7/7 Plus upgrading to iOS 11/12, you can make your shoot JPG again by opening up your iPhone's Settings -> Camera -> Formats -> select Most Compatible, and then the photos you take with your iPhone 7/7 Plus will be in JPG format which could be opened on Mac.
Or use the built-in JPG converter of iPhone to convert HEIC to JPG: Settings -> Camera -> Format -> select Automatic in the Photo tab under "TRANSFER TO MAC OR PC" and then the HEIC photos on your iOS 11 iPhone X/8/8 Plus/7 Plus/7/iPad Pro or iPad 9.7 will be transferred to Mac/PC as JPEG.
Open iPhone HEIC photos with HEIC photo viewer:
As Photos app on Mac powered with macOS sierra 10.12.5 or lower and Windows Photo Viewer can't open HEIC photos at present, help coming from third-party HEIC photo viewers would be your another choice to open iOS 11/12 iPhone HEIC images on computer. Such tools include Apowersoft Photo Viewer, iMyFone Photo Viewer and CopyTrans HEIC for Windows.
Convert HEIC to JPEG/PNG to open on Mac:
Email can also realize the auto conversion if you only have one or a few HEIC photos to be converted, but a HEIC converter like iMazing HEIC converter allows you to batch convert hundreds of HEIC photos as long as you have transferred HIEC photos from iPhone to computer with a certain tool like DearMob iPhone Manager ↓↓↓. Besides, coverting HEIC to other formats with a HEIC converter will loss some image quality to some extent, and you will lose Portrait mode after converting as well.