Failed to decode data coming from client - swift

Question

I am following Ray Wanderlich's book 'Server Side Swift with Vapor' and I am at chapter 26: Adding profile pictures.
First, I defined this struct:
struct ImageUploadData: Content {
var picture: Data
}
Then, in a route I try to decode it:
func postProfilePictureHandler(_ req: Request) throws -> EventLoopFuture<User> {
let data = try req.content.decode(ImageUploadData.self)
...
From the client side, I use Alamofire:
#discardableResult func uploadProfilePicture(for user: User, data: Data) async throws -> User {
enum Error: LocalizedError {
case missingUserID
}
guard let userID = user.id else {
throw Error.missingUserID
}
let appendix = "\(userID)/profilePicture"
let parameters = [
"picture": data
]
return try await withCheckedThrowingContinuation { continuation in
Task {
AF.request(baseUrl + appendix, method: .post, parameters: parameters).responseData { response in
switch response.result {
case .success(let data):
do {
let user = try JSONDecoder().decode(User.self, from: data)
continuation.resume(returning: user)
} catch {
continuation.resume(throwing: error)
}
case .failure(let error):
continuation.resume(throwing: error)
}
}
}
}
}
In my integration tests, I create the picture's data like this:
guard let data = image?.pngData() else {
throw Error.missingPictureData
}
And then I pass it to the above method. The problem is that in the server side, the decoding fails with this error:
The data couldn’t be read because it is missing.
Just to understand if I was doing something else wrong, I tried the above methods with one difference: I replace the type 'Data' with 'String':
struct ImageUploadData: Content {
var picture: String
}
This wouldn't be useful for me because I need a data object, but just as a test to see if this doesn't produce an error, I tried and indeed this is decoded successfully. So I suspect that the problem is in how I encode the data before sending it to the server, but I don't know what's wrong.