MessageBodyWriter not found for media type=multipart/form-data in JerseyTest

I'm trying to create a test for a POST method in a resource class in a Jersey program.

Here's the POST method of the resource:

@POST @Path("/new")
@Consumes(MediaType.MULTIPART_FORM_DATA)
public Response HTTPPostMethod(@FormDataParam("file") InputStream fileIS, 
                           @FormDataParam("file") FormDataContentDisposition contentDispositionHeader) {
    // ... some code that handles the InputStream
}

My ResourceConfig is created in the following way:

public class MyApp extends ResourceConfig {

    public MyApp(String param) {
        register(createMoxyJsonResolver());     
        register(MultiPartFeature.class);
        register(MyResource.class);
    }

     private static ContextResolver<MoxyJsonConfig> createMoxyJsonResolver() {
        final MoxyJsonConfig moxyJsonConfig = new MoxyJsonConfig();
        Map<String, String> nsPrefixManager = new HashMap<String, String>(1);
        nsPrefixManager.put("http://ift.tt/ra1lAU", "xsi");
        moxyJsonConfig.setNamespacePrefixMapper(nsPrefixManager).setNamespaceSeparator(':');
        return moxyJsonConfig.resolver();
    }

    /**
     * Start the Grizzly HTTP Server
     * 
     */
    public final void startHttpServer(int port) {
        try {
            final String url = "http://localhost:" + port + "/myapp";
            final HttpServer server = 
                    GrizzlyHttpServerFactory
                        .createHttpServer(URI.create(url), this);

            Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
                public void run() {
                    server.shutdown();
                }
            }));
            Thread.currentThread().join();

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args)  {
        try {
            final MyApp app = new MyApp(args[1]);
            int port = Integer.parseInt(args[0]);
            app.startHttpServer(port);
        } catch(NumberFormatException e) {
            e.printStackTrace();
        }
    }
}

My jersey test setup:

public class TestBase extends JerseyTest {

    protected Application app;
    protected static final String param = "myparam";

    @Override
    protected Application configure() {
        // The MultiPartFeature is registered.
        this.app = new MyApp(param);
        return app;
    }
}

Finally, the test that's causing problems:

public class MyResourceTest extends TestBase {

// ...

    @Test
    public void testHTTPPost() {

        try {

            FileDataBodyPart filePart = new FileDataBodyPart("file", new File("path/to/a/file/i/know/exists"));
            FormDataMultiPart formDataMultipart = new FormDataMultiPart();
            FormDataMultiPart multipart = (FormDataMultiPart)formDataMultipart.bodyPart(filePart);

            Response result = target("/myResource/new").request().post(Entity.entity(multipart, multipart.getMediaType()));
            formDataMultipart.close();
            multipart.close();

            assertEquals(Response.Status.OK.getStatusCode(), result.getStatus());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

// ...

}

I'm always getting the error:

org.glassfish.jersey.message.internal.MessageBodyProviderNotFoundException: MessageBodyWriter not found for media type=multipart/form-data, type=class org.glassfish.jersey.media.multipart.FormDataMultiPart, genericType=class org.glassfish.jersey.media.multipart.FormDataMultiPart.
    at org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWriterInterceptor.aroundWriteTo(WriterInterceptorExecutor.java:247)
    at org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:162)
    at org.glassfish.jersey.message.internal.MessageBodyFactory.writeTo(MessageBodyFactory.java:1130)
    at org.glassfish.jersey.client.ClientRequest.writeEntity(ClientRequest.java:502)
    at org.glassfish.jersey.client.internal.HttpUrlConnector._apply(HttpUrlConnector.java:388)
    at org.glassfish.jersey.client.internal.HttpUrlConnector.apply(HttpUrlConnector.java:285)
    at org.glassfish.jersey.client.ClientRuntime.invoke(ClientRuntime.java:255)
    at org.glassfish.jersey.client.JerseyInvocation$1.call(JerseyInvocation.java:684)
    at org.glassfish.jersey.client.JerseyInvocation$1.call(JerseyInvocation.java:681)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:315)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:297)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:228)
    at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:444)
    at org.glassfish.jersey.client.JerseyInvocation.invoke(JerseyInvocation.java:681)
    at org.glassfish.jersey.client.JerseyInvocation$Builder.method(JerseyInvocation.java:437)
    at org.glassfish.jersey.client.JerseyInvocation$Builder.post(JerseyInvocation.java:343)
    at services.MyResourceTest.testHTTPPost(MyResourceTest.java:151)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:252)
    at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:141)
    at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:112)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.apache.maven.surefire.util.ReflectionUtils.invokeMethodWithArray(ReflectionUtils.java:189)
    at org.apache.maven.surefire.booter.ProviderFactory$ProviderProxy.invoke(ProviderFactory.java:165)
    at org.apache.maven.surefire.booter.ProviderFactory.invokeProvider(ProviderFactory.java:85)
    at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:115)
    at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:75)

I tried the following code in the TestBase, but it didn't work:

@Override
    protected Application configure() {
        ResourceConfig app = new ResourceConfig();
        app.register(MultiPartFeature.class);
        app.register(MyResource.class);
        this.app = app;
        return app;
    }

Same error. What am I doing wrong here?