Junit mocking files using jmockit

I am trying to test my java code using jmockit for the first time and I am really confused. I have a method that reads a file and returns the line of strings that reads from the file as a list.

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Reader {
	public static final int LIMIT = -1;
	public static final int EMPTY_FILE = 0;
	private String delimiter = ",";

	
	public Reader() {}

	public List<List<String>> readFile(String fileName, String delimiter) throws IOException {
		List<List<String>> rawData = new ArrayList<>();

		File input = new File(fileName);

		if (!delimiter.isEmpty())
			this.delimiter = delimiter;

		if (input.length() == EMPTY_FILE) {
			throw new IOException("File is empty. Check file and try again.");
		}

		BufferedReader reader = new BufferedReader(new FileReader(input));

		String line;

		while ((line = reader.readLine()) != null) {
			List<String> lineData = Arrays.asList(line.split(this.delimiter, LIMIT));
			rawData.add(lineData);
		}
		return rawData;
	}
}

I am trying to test this code using mocked readers and bufferedReader but without any luck. Obviously I am doing something wrong but I can't figure out how to do it properly.

What I want is to create a mocked file that will be read and test it like its empty or non empty.

What I have tried so far :

class ReaderTest {
	static final String FILENAME = "input.txt";

	@Injectable
	File mockedFile;

	@Mocked
	BufferedReader mockedBufferedReader;

	@Mocked
	FileReader mockedFileReader;

	@Test
	void readNonEmptyInputFileShouldDoNothing() throws FileNotFoundException {
		new Expectations(File.class) {{
			new File(anyString);
			result = mockedFile;
		}};

		new Expectations(BufferedReader.class) {{
			new FileReader(anyString);
			result = mockedFileReader;

			new BufferedReader(mockedFileReader);
			result = mockedBufferedReader;
		}};
		Reader reader = new Reader();

		Assertions.assertDoesNotThrow(() ->
				reader.readFile(FILENAME, FieldsConstants.DELIMITER));
	}
}

This test gives me an IllegalArgumentException error:
Invalid Class argument for partial mocking (use a MockUp instead): class java.io.File

I managed to solve my issue using PowerMock and EasyMock API. The only issue i faced was that in the beginning i was using Junit5 and PowerMock wasn't working as intended. I switched to Junit4 and everything worked out just fine. Just some sample code if anyone is interested:

@RunWith(PowerMockRunner.class)
@PrepareForTest({Reader.class})
@PowerMockIgnore({"javax.management.*", "javax.script.*"})
public class ReaderTest {
	public static final String TEST_LINE = "2,Ned,Flanders,SELLER,,,LINE_A,Springfield,12.8,TRUE";
	public static final String FILENAME = "file_name";
	public static final int LIMIT = -1;

	private static List<List<String>> tmpList;

	@BeforeClass
	public static void beforeAll() {
		List<String> tmpLine = Arrays.asList(TEST_LINE.split(FieldsConstants.DELIMITER, LIMIT));
		tmpList = new ArrayList<>();
		tmpList.add(tmpLine);
	}

	@Test
	public void readNonEmptyInputFileShouldDoNothing() throws Exception {
		File mockFile = createMockAndExpectNew(File.class, FILENAME);
		FileReader mockFileReader = createMockAndExpectNew(FileReader.class, mockFile);
		BufferedReader mockBufferedReader = createMockAndExpectNew(BufferedReader.class, mockFileReader);

		Reader reader = new Reader();

		expect(mockFile.length()).andReturn((long) 100);

		expect(mockBufferedReader.readLine()).andReturn(TEST_LINE);
		expect(mockBufferedReader.readLine()).andReturn(null);

		replayAll();

		assertEquals(reader.readFile(FILENAME, ""), tmpList);
		verifyAll();
	}
}