Swift 3 and iOS: Form data and multipart uploads with URLRequest

If you want to upload an image to a server, an image from the photo gallery or camera for example, you often use a multipart request.

First create a POST Request as normal, with with a boundary string created via a UUID request and the content type as multipart/form-data with the boundary string.

var r  = URLRequest(url: URL(string: "https://prospero.uatproxy.cdlis.co.uk/prospero/DocumentUpload.ajax")!)
r.httpMethod = "POST"
let boundary = "Boundary-\(UUID().uuidString)"
r.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")

We must now create the http body. We use the below function, passing in parameters as a dictionary with strings, the boundary string we created, data from the UIImage, a mime-type and the filename for the image.

r.httpBody = createBody(parameters: params,
                        boundary: boundary,
                        data: UIImageJPEGRepresentation(chosenImage, 0.7)!,
                        mimeType: "image/jpg",
                        filename: "hello.jpg")

The createBody method first loops over the parameters dictionary, adding them to the body as a Content-Disposition with the boundary.

Finally, it adds the image as data, with the filename, the mime-type and with the boundary as before.

func createBody(parameters: [String: String],
                boundary: String,
                data: Data,
                mimeType: String,
                filename: String) -> Data {
    let body = NSMutableData()

    let boundaryPrefix = "--\(boundary)\r\n"

    for (key, value) in parameters {
        body.appendString("Content-Disposition: form-data; name=\"\(key)\"\r\n\r\n")

    body.appendString("Content-Disposition: form-data; name=\"file\"; filename=\"\(filename)\"\r\n")
    body.appendString("Content-Type: \(mimeType)\r\n\r\n")

    return body as Data

The appendString doesn’t exist on a NSMutableData. It’s a helper extension as defined below:

extension NSMutableData {
    func appendString(_ string: String) {
        let data = string.data(using: String.Encoding.utf8, allowLossyConversion: false)

Now we have the URLRequest we can send it off as usual.

Posted: Tuesday 22 November 2016, 10:5AM

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