GraphicsMagick++ expert help needed

[John W. Eaton]

See the following bug report about imwrite failing when an image
format is specified with an arbitrary filename:

  https://savannah.gnu.org/bugs/?30784

The problem appears to be that Octave is calling

  Magick::writeImages (imvec.begin (), imvec.end (), filename);

to write the image, and this function does not include an explicit
image storage type so it depends on the filename to determine the
type.  If I knew which Magick++ function(s) to call to write an image
to an arbitrary filename with a specified imagee type, I could
probably fix this.  But I have no clue.  Are there any
GraphicsMagick++ experts out there who can help with this problem?

Thanks,

jwe