it is not possible when using only the --request option because it would be somehow difficult to difference between real string content and file paths inside the file, but you can use jsreport.exe render --template.content test.html --template.engine handlebars --template.recipe phantom-pdf --data report.json --out file.pdf to do what you want.
also just FYI you can run jsreport.exe render -h to see some examples about the usage of the command, and descriptions about the options 😃