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So this was interesting. Like Jeremy Wootten mentioned what you can put behind a .contract Exec is limited. So calling a shell script is part of the solution. So step one is creating something like the following contract file:
~/.local/share/contractor:
[Contractor Entry]
Name=Resize to 1200px
MimeType=image;
Exec=resize-image-1200.sh %f
Gettext-Domain=...
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