Article.java

@MultipartBody
public class Article {
    String author;
    File photo;
}

MultipartBody.java

@Target(TYPE)
@Retention(RUNTIME)
public @interface MultipartBody {
}

MultipartConvertor.java

public class MultipartConvertor implements Converter {

    private final Converter delegateConvertor;

    public MultipartConvertor(Converter gsonConverter) {
        this.delegateConvertor = gsonConverter;
    }

    @Override public Object fromBody(TypedInput body, Type type) throws ConversionException {
        return delegateConvertor.fromBody(body, type);
    }

    @Override public TypedOutput toBody(Object object) {
        Class<?> rawType = object.getClass();
        if (rawType.isAnnotationPresent(MultipartBody.class)) {
            List<Field> fields = ReflectionUtil.getFields(rawType);
            MultipartTypedOutput multipartTypedOutput = new MultipartTypedOutput();
            for (Field field : fields) {
                if (field.getClass().equals(String.class)) {
                    try {
                        String value = (String) field.get(object);
                        TypedString typedString = new TypedString(value);
                        multipartTypedOutput.addPart(field.getName(), typedString);
                    } catch (IllegalAccessException e) {
                        // TODO: handle
                    }
                } else if (field.getClass().equals(File.class)) {
                    try {
                        File value = (File) field.get(object);
                        TypedFile typedFile = new TypedFile("todo", value);
                        multipartTypedOutput.addPart(field.getName(), typedFile);
                    } catch (IllegalAccessException e) {
                        // TODO: handle
                    }
                }else {
                    throw new UnsupportedOperationException("Not implemented: " + field.getClass());
                }
            }
            return multipartTypedOutput;
        }
        return delegateConvertor.toBody(object);
    }
}

Retrofit.java

public interface Retrofit {
    @POST("/api/v1/articles/")
    public Observable<Response> createArticle(@Body Article article);
}