set the file type parameter after reading the image

set the file type parameter after reading the image

imagemagick, when reading an image, sets its magick parameter to
indicate the file type, overriding the explicitly set file type for
output if it is set at creation.

as a result, previously (with graphicsmagick-libmagick-dev-compat
1.3.18-1 providing Image::Magick), svg output files were not png,
neither svg, but mvg (imagemagick vector graphics).