May I suggest that as this is a thread about the Beta Photos App, discussion on what file formats Plex supports at a server level goes in a different thread. For example the one @CaptainKen2_1 has already pointed at: Feature Suggestion - HEIC/HEIF support in photo libraries (2017).
Yes, I know, clearly if nothing has happened in that thread for nearly 10 years its unlikely that anything will change now. HEIC is part of the HEVC patent though, so now that Plex supports H.265 (and sometimes H.266) there may be some legal headway. But in the case of someone who simply cannot save photos as .jpg there are other solutions (e.g Immich).
As an aside though, .mov is presumably supported as it is a video format. Plex is an offshoot from XBMC which was primarily for video. Ironically I see that even Kodi (what XBMC became) does not support HEIC natively and uses an add-on. ¯\(ツ)/¯
