fix(iOS): Ensure MultiUploadTask filename is always a list

The `Uploader.swift` was incorrectly modifying the `filename`
property of a `MultiUploadTask` to a single string when only one file
was present. This caused a JSON deserialization error on the Dart side,
as `MultiUploadTask.fromJson` expects a list of strings.

This commit adds a check to ensure that the `filename` property is only
modified for regular `UploadTask`s, preserving the list format for
`MultiUploadTask`s regardless of the number of files.

Fixes #510

Author: 781flyingdutchman

ios/background_downloader/Sources/background_downloader/Uploader.swift

@@ -92,9 +92,8 @@ public class Uploader : NSObject, URLSessionTaskDelegate, StreamDelegate {
                 return false
             }
             let derivedFilename = path.components(separatedBy: "/").last!
-            if filesData.count == 1 {
+            if filesData.count == 1 && task.taskType != "MultiUploadTask" {
                 // only for single file uploads do we set the task's filename property
-                
                 let newTask = task.copyWith(filename: maybeFileUri != nil
                                             ? pack(filename: derivedFilename, uri: maybeFileUri!)
                                             : derivedFilename)