Jenkins email-ext not attaching HTML file to body

I'm trying to send out an email when my Jenkins build ends. The HTML file gets generated in a previous step and I have verified its location.

This is the error that's being generated [Error replacing 'FILE' - Error processing tokens]

always  {
emailext (mimeType: 'text/html',
            body: '${FILE, path="report_stage.html"}',
            subject: currentBuild.currentResult + " : " + env.JOB_NAME,
            to: "${params['On Failure Email List']}"
            )
}