Apple is now using HEIF file format for iMessage image attachments by default, so copying attachments from iMessage conversations causes the attachment to uploaded but fail to render because these aren't supported by any browsers.
It's called HeifToJpeg because HEIF is the image container format. Often .heic is the file extension, but HEIF could also be .avif or .heif. I don't know where those can be found in the wild except on some bleeding edge cameras, so I don't have any real world examples to test with. It would be uncommon to upload those as you'd probably grade them first and export to JPEG. It seemed better to name it after the container so we can add more automatic conversion of formats later.