my latest photos are all showing as HIEC files

Here is a bit background reading on HEIF: Using HEIF or HEVC media on Apple devices - Apple Support

To be able to work with media in the high efficiency format HEIC and HEVC on your Mac you need to upgrade to macOS 10.13 or macOS 10.14. If you cannot do that, change the setting on the iPhone. There are two settings that are relevant:

• The Settings > Camera determine, if the the Camera is staoring the photos as HEIC or JPEG,
• The Settings > Photos > Transfer to Mac or PC detemine, if the photos will be converted to JPEG, when you download them from the iPhone.

You can convert the HEIC images you already downloaded from the iPhone as HEIC toJPEGS using the online converter:

Check your iPhone, Settings/Camera/Formats to see what is set for the format of photos. If you don't want HIEC then select "Most Compatible".