Add a real .HDR export file, not .DNG

Hello @Hodak 

Please see this article for more info: https://helpx.adobe.com/lightroom-cc/using/hdr-ios.html 

In short, while DNG can contain more image data than other formats, neither DNG or TIF are file types in which LR encodes HDR info.  For true HDR display, one must export as AVIF or JXL.  Support for these formats is growing, but does not yet exist everywhere.  Also, if you export an HDR as a JPG, gainmap info is encoded, but not all platforms (iOS) or apps support Adobe's gainmap standard.

Here's a breakdown of support on iOS:

• On iOS 15, AVIF and JXL are not supported by the camera roll. (Also, no apps on iOS 15 can display HDR.)
• On iOS 16, you can save AVIF (but not JXL) to the camera roll and have it be visible, but Photos.app won't show the HDR (If it displays, there may be clipping. Also, it's unclear if other apps would support importing such photos.)
• On iOS 17, you can save AVIF and JXL photos to the camera roll, and Photos.app will show them in HDR. However, support by third-party apps is still an open question (iOS 17 adds OS support that other apps can use to support HDR, but it will take time before other apps add this support).
• No version of iOS, even iOS 17, will support the HDR gain map data in LR exported JPEG files from our recent release. In the future, industry partners/platforms will likely support this once it approaches being an ISO standard. For now, Lr Mobile writes Adobe Camera Raw gain maps into our JPEGs on that will display as Standard Dynamic Range images on unsupported platforms.

Note: we'll soon be posting updates to the help article above to clarify export & display expectations.