Retrofit 2.0-beta-2 is adding literal quotes to MultiPart values

This is because it’s running through the JSON converter.

Solution1:
use RequestBody instead of String

public interface ApiInterface {
    @Multipart
    @POST("user/login/")
    Call<SessionToken> userLogin(@Part("username") RequestBody username, @Part("password") RequestBody password);
}

Build RequestBody:

RequestBody usernameBody = RequestBody.create(MediaType.parse("text/plain"), usernameStr);
RequestBody passwordBody = RequestBody.create(MediaType.parse("text/plain"), passwordStr);

Launch network operation:

 retrofit.create(ApiInterface.class).userLogin(usernameBody , passwordBody).enqueue()....

Solution2: Create a custom ConverterFactory to dispose String part value.

For: Retrofit2 final release not beta. (com.squareup.retrofit2:retrofit:2.0.0)

Create your StringConverterFactory:

public class StringConverterFactory extends Converter.Factory {
private static final MediaType MEDIA_TYPE = MediaType.parse("text/plain");

public static StringConverterFactory create() {
    return new StringConverterFactory();
}

@Override
public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations, Retrofit retrofit) {
    if (String.class.equals(type)) {
        return new Converter<ResponseBody, String>() {

            @Override
            public String convert(ResponseBody value) throws IOException {
                return value.string();
            }
        };
    }
    return null;
}

@Override
public Converter<?, RequestBody> requestBodyConverter(Type type, Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit) {
    if(String.class.equals(type)) {
        return new Converter<String, RequestBody>() {
            @Override
            public RequestBody convert(String value) throws IOException {
                return RequestBody.create(MEDIA_TYPE, value);
            }
        };
    }

    return null;
}

}

Add to your retrofit instance:

retrofit = new Retrofit.Builder()
            .baseUrl(SERVER_URL)
            .client(client)
            .addConverterFactory(StringConverterFactory.create())
            .addConverterFactory(GsonConverterFactory.create())
            .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
            .build();

Attention: StringConverterFactory should add before GsonConverterFactory!

then you can use String as part value directly.

You can find more information about this issue in https://github.com/square/retrofit/issues/1210