Just right click your .heif/ .heic image and select "Convert this image to JPEG". It converts it to a jpeg file using the same location.
Needs the PHP Imagick extension to be installed on your host compiled with support for the HEIC/ HEIF format. Check if your PHP installation supports the HEIC format by entering "php -i" in the terminal and go to the Imagick section.
Here you can find a good tutorial about how to compile Imagemagick with HEIC support and install the PHP extension.
The official Docker Nextcloud image already includes the Imagick extension with support for the HEIC format.