I found - running the test suite for dragonfly-libvips - that there are several image formats which are not supported by ForeignSaveBuffer.

The formats are avif, bmp, dz, gif, hdr, heic, heif, jpc, jpt, jp2, j2c, j2k and webp.

dragonfly_libvips typically processes an image (resize, crop, encode), then returns it to the caller after writing the new image to a buffer so:

 img.write_to_buffer(".#{format}", **DragonflyLibvips.symbolize_keys(**output_options)),
        'name' => "temp.#{format}",
        'format' => format

I'm not familiar with all the image formats listed, but I can imagine wanting to handle webp or heic.
At the moment I have added them to a list of unsupported formats but I wonder if any, all or some should be added to foreign.

thanks