How to save Image item as jpg?

Hello, following the example : https://github.com/hluk/copyq-commands/blob/dec292d8fddc6ec21f649ac7154802069fde75ab/Application/save-item-clipboard-to-file.ini
I can save copied image as png file, but failed when try to save as jpg, I Noticed that all Images in CopyQ are stored with MIME type 'image/png'
I tried follow API to convert png data to jpg, but to no avail:
read('image/jpg',currentitem())
setData('image/jpg', read('image/png',currentitem()))
change(currentitem(), 'image/jpg', data('image/png'))
so how can I convert an Item only with 'image/png' MIME type available to 'image/jpg' then save as a jpg file?
many thanks.