How to maintain HEIF format for images generated by PHAssetCreationRequest

I generated an asset in the photolibrary by adding the unedited image, adjustmentData, and edited image with PHAssetCreationRequest.addResouce(). The image is saved in the photolibrary as HEIF. (Check with the photolibrary)

Then, when I save the image generated with PHAssetCreationRequest.addResouce() to the Files app, it becomes JPEG. On the other hand, even if I edit an image taken with the camera and save it to the Files app, it still maintains the HEIF.

Do you know why this happens?

Also, how can I maintain the HEIF even when saving it in the Files app?

Thanks.