JPEG to HEIF

Apple introduced HEIF on iOS devices in September 2017 with iOS 11 on mobile devices and macOS 10.13 macOS High Sierra on the Mac. As you can see in linked documents provided by K Shaffer the Setting > Camera > Formats on your iPhone determines , if the Camera will save new photos in the space saving High Efficiency format HEIF or or as larger JPEG files for more compatibility. when now all new files are in the High Efficiency format, the setting "Camera > Formats" may have been changed from "more compatible" to "High Efficiency" when you installed a system upgrade. System upgrades will sometimes change the Settings if the default has become changed.

You may want to change the Settings > Camera > Format back to "Most Compatible", if you prefer JPEG to HEIF, but then your new photos and videos will need twice the storage on the device and in iCloud. I prefer HEIF because of the better compression at the same quality.

What I cannot recommend to do is to change the Settings > Photos > Transfer to Mac or PC to "Most Compatible". With this setting all HEIC files will we converted to JPEG or H.264 when you download them to a computer. But it will disable the duplicate detection when importing the photos to from the iPhone to Photos for Mac. So you will have to select the new photosto be imported manually and cannot rely on the duplicate detection to prevent already imported files from being imported again.

An article suggests some means to revert to a default..

Whether or not HEIC is the same as HEIF? Another story.

How to take JPEG photos on your iPhone instead of HEIC | iMore

https://www.imore.com/how-take-jpeg-pictures-ios-instead-heic

How to Make Your iPhone Use JPG and MP4 Files Instead of HEIF, HEIC - howtogeek

https://how-to-make-your-iphone-use-jpg-and-mp4-files-instead-of-heif-and-hevc/

How to avoid HEIC format when transferring photos from your iPhone - appletoolbox

https://appletoolbox.com/how-to-avoid-heic-format-when-transferring-photos-from-your-iphone/

This other item, was found by accident, not really helpful.

Using HEIF or HEVC media on Apple devices - Apple Support

//support.apple.com/en-us/HT207022

Seems to be 'defaulting' to some type of video codec

while this support article, says nothing of changing it.

..On a Mac you could just change file name to convert

automatic, then JPEG would open it, instead of HEIF..

~ But in the iOS world, some things can be/seem odd.

Depending on the iOS version and recent secure updates

there may have been a new change for default to later iOS.

Perhaps through more related details about your device

and its present operating system (update can change it)

..then those interactions may help present an answer.

Linked articles probably cover an appropriate possible fix.

..And knowing the system build version (and Mac) helps..

As a "Volunteer" without compensation, my time is limited.