Photos plugins: use TIFF instead of JPG as interchange format
Short answer : If the plugin has no option to select "TIFF", you'll have to live with JPEG

Long answer: When you look at the example source code that apple provides for Photos plugins (Photos sample code for plugins), you can see that the file types for the output are explicitly set to kUTTypeJPEG by

            guard let destination = CGImageDestinationCreateWithURL(output.renderedContentURL as CFURL, kUTTypeJPEG, 1, nil)


This means that the plugin decides the format it writes to. If the format is hardcoded and the plugin does not give you the choice to change the output format to TIFF, you will have to live with its choice. If it allows you to write to kUTTypeTIFF, you get what you want.

Hope this helps you.