RestAssured + multipart/form-data

restAssuredMultipart.java

// Nota bene: Add @JsonIgnore to MultiPart fields

public static final ObjectMapper MAPPER = new ObjectMapper()
    .configure(FAIL_ON_UNKNOWN_PROPERTIES, false)
    .configure(FAIL_ON_EMPTY_BEANS, false);

public static final TypeReference<Map<String, Object>>
    MAP_TYPE_REF = new TypeReference<>() {};

public static Map<String, Object> toMap(Object obj) {
    return MAPPER.convertValue(obj, MAP_TYPE_REF);
}

public static final Converter<String, String> TO_SNAKE_CASE
    = LOWER_CAMEL.converterTo(LOWER_UNDERSCORE);

@SneakyThrows
public static RequestSpecification givenMultiPartRequest(Object obj) {
    var spec = given().formParams(toMap(obj));
    for (var method : obj.getClass().getDeclaredMethods()) {
        var ctrl = method.getName();
        if (!ctrl.startsWith("get"))
            continue;
        ctrl = ctrl.replace("get", "");
        ctrl = toLowerCase(ctrl.charAt(0)) + ctrl.substring(1);
        var type = method.getReturnType();
        if (MultipartFile.class.isAssignableFrom(type)) {
            var mp = (MultipartFile) method.invoke(obj);
            if (mp != null)
                spec.multiPart(TO_SNAKE_CASE.convert(ctrl), mp.getName(), mp.getBytes());
        } else if (Collection.class.isAssignableFrom(type)) {
            var coll = (Collection<?>) method.invoke(obj);
            if (coll != null && !coll.isEmpty() && MultipartFile.class
                    .isAssignableFrom(coll.toArray()[0].getClass()))
                for (var elt : coll) {
                    var mp = (MultipartFile) elt;
                    spec.multiPart(TO_SNAKE_CASE.convert(ctrl), mp.getName(), mp.getBytes());
                }
        }
    }
    return spec;
}