Power mock to test a file reader?

I am trying to access a function call inside a method so I can test the FileReader sonar is highlighting. I need to test the transform() method. I want to high jack the calls inside that method. So, I can pass in a mock file.

 Reader gmFile = createFileReader(getGmFile());

Here is my test function which doesnt do what i want.

@Test
public void testFileReader() throws Exception {

    File gmFile = mock(File.class);
    File smsFile = mock(File.class);
    File outFile = mock(File.class);


    //execute
    SmsTransformationImpl sms = new SmsTransformationImpl(gmFile, smsFile, outFile);
    Reader reader = Whitebox.<Reader>invokeMethod(sms, "createFileReader", smsFile);
    //Reader gmFile = createFileReader(getGmFile());

}

Here is the whole class.

public class SmsTransformationImpl implements SmsTransformation {

private final File gmFile;
private final File smsFile;
private final File out;

/**
 * Constructor to set the files
 *
 * @param gm  gmFile
 * @param sms smsFile
 * @param out outFile
 */
public SmsTransformationImpl(File gm, File sms, File out) {
    if (gm == null || sms == null || out == null) {
        throw new IllegalArgumentException("Files have no content");
    }

    this.gmFile = gm;
    this.smsFile = sms;
    this.out = out;
}

/**
 * This method return the sms file object
 *
 * @return smsFile
 */
public File getSmsFile() {
    return smsFile;
}

/**
 * This method returns the gm file object
 *
 * @return gmFile
 */
public File getGmFile() {
    return gmFile;
}

/**
 * This method returns the out file object
 *
 * @return outFile
 */
public File getOutFile() {
    return out;
}

/**
 * This function uses file reader to read in the files
 * ready to write
 */
@Override
public void transform() throws IOException {
    try (
            Reader gmFile = createFileReader(getGmFile());
            Reader smsFile = createFileReader(getSmsFile());
            Writer outFile = createFileWriter(getOutFile());
    ){
        transform(gmFile, smsFile, outFile);
    }
}

/**
 * @param gm  gm file
 * @param sms sms file
 * @param out out file
 * @throws IOException
 */
public static void transform(Reader gm, Reader sms, Writer out) throws IOException {
    writeStream(out, gm);
    writeStream(out, sms);
}

private static void writeStream(Writer writer, Reader reader) throws IOException {
    int count = 0;
    char[] buffer = new char[1024];

    while ((count = reader.read(buffer)) != -1) {
        writer.write(buffer, 0, count);
    }
}

private Reader createFileReader(File file) throws IOException {
    return new FileReader(file);
}

private Writer createFileWriter(File file) throws IOException {
    return new FileWriter(file);
}

}

How can use PowerMock to call transform().createFileReader(mock file); to pass in a mock File.