The Mirth channel is setup to to look for the configuration files in the folder c:\temp\OptionalSettings and is named [Customer Number]-[Template Id].json
Here is a sample of what the settings file will support:
where the key name, email corresponds to the output settings name in NFS.
rasterizePdf turns on and off the rasterizing the pdf. When set to true, the pdf is rasterized and then removed from the file list to be sent.
imageType is the type of image that will be produced. I am not sure what all is supported but I had tried png, jpg and tif and it all worked.