Past discussion from 2020, with no apparent resolution: hxxps://discourse.joplinapp.org/t/image-type-heic/13050
For over 5 years, iOS 11+ devices have saved photos in HEIC format to conserve storage space. This means that when you AirDrop a photo from your iPhone to your Mac, it will be an .HEIC file rather than a .JPG.
Joplin currently does not render .HEIC files, so if you drop such a photo into a note it will only be visible if you open it with a third party program.
Since iOS is by far the most popular mobile platform in America and HEIC is the default setting on iOS, would it be possible to add logic to Joplin that renders these images?
FWIW, if I install ImageMagick with homebrew, I can run
convert x.heic y.jpg from the command line. So this functionality is readily available in a popular open source library.