Resulting image format is determined by filename extension, unknown
extension will result in error.
Image loaded and displayed by castle-image-viewer is internally always stored
in format comfortable for OpenGL. This includes many formats, but not RGBE.
It means that if you will load RGBE image to castle-image-viewer and then
you will save it (even to the RGBE format again)
then you loose RGBE precision (and clamp color values above 1.0).
Also, S3TC compressed images (from KTX, DDS files) will be always decompressed,
and saving them back will always make uncompressed files.