Alamofire changes multipart/form-data encoding depending on input - ios

Question

Working on an iOS project I found out that Alamofire changes the Content-Transfer-Encoding of a multipart/form-data field depending on the field value.
Goal
Update an image with some informations (unicode strings) to an endpoint.
Details
I define the form's fields values as follows:
let formFields: [String: String] = [
"key": _model
]
I upload the entire form (image + informations) as follows:
Alamofire.upload(
multipartFormData: { multipartFormData in
// 1. The form's fields
for (key, value) in formFields {
guard let byteString = value.data(using: String.Encoding.utf8, allowLossyConversion: false) else { continue }
multipartFormData.append(byteString, withName: key)
}
// 2. The picture
let fileName = "file.jpeg"
multipartFormData.append(pictureData, withName: ParameterKeys.BikeImage, fileName: fileName, mimeType: "image/jpeg")
},
to: _url,
method: .post,
encodingCompletion: { encodingResult in
switch encodingResult {
case .success(let upload, _, _): /* Called when the encoding succeds. */
upload.validate(statusCode: 200..<300)
upload.uploadProgress { progress in // main queue by default
progressHandler(progress)
}
upload.response { (response) in
responseHandler(response)
}
case .failure(let error): /* Called when the encoding fails. */
failureHandler(error)
}
}
)
Behaviour 0
_model is "Standard" ✅
multipartFormData.append(byteString, ...) appends "5374616e64617264" in the form ✅
The server gets "Standard" without Content-Transfer-Encoding ⁉️
Behaviour 1
_model is "Ström Bike" ✅
multipartFormData.append(byteString, ...) appends "537472c3b66d2042696b65" in the form ✅
The server gets "Str=C3=B6m Bike" with Content-Transfer-Encoding: quoted-printable ⁉️
Behaviour 2
_model is "Abräcadabra" ✅
multipartFormData.append(byteString, ...) appends "416272c3a463616461627261" in the form ✅
The server gets "QWJyw6RjYWRhYnJh" with Content-Transfer-Encoding: base64 ⁉️
Conclusion
On the server, I cannot determine the format of the form data. Is there a way to specify the type of encoding to use for the form fields?
Thank you in advance.