Here’s a screenshot
I think that the image viewer widget and similar are limited to typical image types like JPEG and PNG. You would have to create your own HEIC-Image-Viewer widget. Can't help you with that unfortunately.
Is there a specific issue you are running into? Mendix places no restrictions on file types: Mendix treats all files as binary data and simply stores that for later downloading. Showing HEIC files in certain browsers may be an issue, but a user can download the image and use another app to view the image.
Can you try to convert the image to JPG and store it in the Mendix File document. Find the link below for reference.