Upload a zip file in a background URLSession.

I'm trying to upload an existing zip file to a server, but the file itself doesn't seem to be present in the request when I check what the server actually receives. Here's my current implementation:

func uploadZip(filePath: URL, authorization: String, callback: @escaping ((Bool) -> Void)) {
    BGUploadManager.callback = callback
    let url = URL(string: "\(APINetwork.BASE_URL)")!
        
    var request = URLRequest(url: url)
    request.headers = ["Authorization": "Bearer " + authorization]
    request.method = HTTPMethod(rawValue: "POST")
    request.setValue("application/zip", forHTTPHeaderField: "Content-Type")
        
    let config = URLSessionConfiguration.background(withIdentifier: "identifier")
    config.isDiscretionary = false

    let session = URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue.main)
        
    let task = session.uploadTask(with: request, fromFile: filePath)
    task.resume()
}

There are implemented handlers for completion and progress and they work properly. The problem is really just the actual file not being sent (or if it is I don't know how to get it). Here's parts of what the server receives:

headers:
   { host: 'localhost:1337',
     'content-type': 'application/zip',
     'accept-encoding': 'gzip, deflate',
     connection: 'keep-alive',
     accept: '*/*',
     'user-agent': 'App/37 CFNetwork/1237 Darwin/20.4.0',
     authorization:
      'Bearer sometokenhere',
     'content-length': '89634' }
body: {}

Checking for request.files also shows that it's empty. The Content-Length value is correct. The filePath is the full path and does actually point to the zip file.