Whenever I try to use a parameter from a JSON file I'm only getting the name of the temp file that is copied to the workspace. Here is an example of usage and output:

configFileProvider([configFile(fileId: 'testFile', variable: 'TESTPARAM')]) {
bat "echo $env.TESTPARAM"
}

The output from this is: 
C:\jenkins\TrustJenkinsPipeline\workspace@tmp\config4843133872643832765tmp

I have tried all of the following ways as well:
bat "echo $TESTPARAM"
bat "echo ${TESTPARAM}"
bat "echo ${env.TESTPARAM}"
bat "echo %TESTPARAM%"
bat "echo %env.TESTPARAM%"

All of them either return nothing or the same output as above.

Here is the contents of 'testFile':
{
"TESTPARAM": "testValue"
}

I do not know if this is a bug or me doing it wrong, but so far I haven't had any luck figuring it out.