Sync uploads the photos as they are saved on the device. With the release of iOS 11, Apple replaced JPEG with a new image format called HEIC.
HEIC is the file format name Apple has chosen for the new HEIF (High Efficiency Image Format) Standard. Using advanced and modern compression methods, it allows photos to be created in smaller file sizes while retaining a higher image quality compared to JPEG/JPG.
This new file type, however, may not be compatible with all devices. Even earlier versions of OS X (pre-High Sierra) will not be able to open HEIC files by itself.
To set your Apple devices to take photos in JPEG/JPG instead of HEIC: