Google cloud Endpoint REST API - call with multiples arguments - api

Question

I have to make a small project for school, and I have to create and use an API Rest with google endpoint.
I have created my method:
image corresponds to the string obtained with the method new FileReader().readAsDataURL used in my frent-end on an image.
user corresponds to a google account (using the credential).
#ApiMethod(name = "createNewPost", httpMethod = HttpMethod.GET)
public void createNewPost(#Named("image") String image, User user) throws
UnauthorizedException, EntityNotFoundException {
if (user == null) {
throw new UnauthorizedException("Invalid credentials");
}
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Calendar date = Calendar.getInstance();
//Creates the post with relevant data
Entity tinyPost = new Entity("Post");
tinyPost.setUnindexedProperty("User", user.getId());
tinyPost.setUnindexedProperty("Image", image);
datastore.put(tinyPost);
//Creates the post index containing all receiver and linked with the post
Entity tinyPostIndex = new Entity("PostIndex", (3000 - date.get(Calendar.YEAR)) + ":" +
(12 - date.get(Calendar.MONTH)) + ":" +
(31 - date.get(Calendar.DAY_OF_MONTH))+ ":" +
(24 - date.get(Calendar.HOUR_OF_DAY))+ ":" +
(60 - date.get(Calendar.MINUTE))+ ":" +
(60 - date.get(Calendar.SECOND)));
tinyPostIndex.setUnindexedProperty("Post", tinyPost.getKey());
tinyPostIndex.setProperty("Receiver", datastore.get(KeyFactory.stringToKey(user.getId())).getProperty("Subscribers"));
datastore.put(tinyPostIndex);
}
So I use the url to make my request:
"_ah/api/myApi/v1/createNewPost" + '?image=' + reader.result + '&access_token=' + Login.ID
The problem is that I get a 400 error every time.
I tried replacing reader.result with a regular string, but it doesn't change anything.
I have another method with only User as parameter and it works fine
"_ah/api/myApi/v1/registerUser" + '?access_token=' + Login.ID
Thanks