Soon we will officially support .hgt.zip files (see !2483) but in fact,
even without this, the file-compressor actually support any other
formats we support, compressed into one of the container formats
supported by this plug-in.
This fixes such error message:
> gimp_image_set_file: failure to set file 'file:///home/jehan/Documents/gimp-splash-deleteme.png.gz' with extension 'gz'. The extension must be '.xcf' (or a compressed variant).
3a698a05a1