Super simple way to queue up files to be uploaded to your S3 bucket in Unity without relying on the AWS .NET APIs

AWSUploadAsync.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Threading;
using System.Security.Cryptography;
using System.Text;
using UnityEngine;

public class AWSUploadAsync : MonoBehaviour
{
    // Adapted from:
    // https://medium.com/xrlo-extended-reality-lowdown/uploading-to-aws-from-unity-5e7de2c80fce

    // TODO: Point this to your server
    private const string awsBucketName = "bucketName";
    private const string awsAccessKey = "XXXXXXXXXXXXXXXXXXXX";
    private const string awsSecretKey = "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy";
    private static string awsURLBaseVirtual = "";
    
    [SerializeField]
    private bool disableUploads;
    
    public bool Uploading { get; private set; }
    
    public class Upload
    {
        public Thread Thread { get; private set; }
        public string FileName;
        public string FilePath;
        public bool Done { get; private set; }
        public bool Fail { get; private set; }

        private Queue<Upload> _uploads;

        public void StartUpload()
        {
            if (Thread == null)
            {
                Debug.Log("Starting upload thread");
                Thread = new Thread(() => UploadFileToAWS3(FileName, FilePath));
                Thread.Start();
            }
        }
        
        private void UploadFileToAWS3(string fileName, string filePath)
        {
            Done = false;
            Fail = false;
            
            // TODO: You may or may not need this.
            // Thread.Sleep(1000);
            
            string currentAWS3Date =  
                System.DateTime.UtcNow.ToString(
                    "ddd, dd MMM yyyy HH:mm:ss ") +
                "GMT";
            string canonicalString =
                "PUT\n\n\n\nx-amz-date:" +
                currentAWS3Date + "\n/" +
                awsBucketName + "/" + fileName;
        
            UTF8Encoding encode = new UTF8Encoding();
            HMACSHA1 signature = new HMACSHA1();
        
            signature.Key = encode.GetBytes(awsSecretKey);
            byte[] bytes = encode.GetBytes(canonicalString);
            byte[] moreBytes = signature.ComputeHash(bytes);
            string encodedCanonical = Convert.ToBase64String(moreBytes);
            string aws3Header = "AWS " +
                                awsAccessKey + ":" +
                                encodedCanonical;
            string URL3 = awsURLBaseVirtual + fileName;

            var requestS3 = (HttpWebRequest)WebRequest.Create(URL3);
        
            requestS3.Headers.Add("Authorization", aws3Header);
            requestS3.Headers.Add("x-amz-date", currentAWS3Date);
        
            byte[] fileRawBytes = File.ReadAllBytes(filePath);
        
            requestS3.ContentLength = fileRawBytes.Length;
            requestS3.Method = "PUT";
            requestS3.Timeout = 100000;
            requestS3.KeepAlive = false;

            Debug.Log("[Thread] Starting upload: " + fileName);
            
            try
            {
                using (var s3Stream = requestS3.GetRequestStream())
                {
                    s3Stream.Write(fileRawBytes, 0, fileRawBytes.Length);
                }
                Debug.Log("[Thread] Sent bytes: " +
                          requestS3.ContentLength +
                          ", for file: " +
                          fileName);
            }
            catch (Exception e)
            {
                Debug.Log("[Thread] Upload failed." + e);
                Fail = true;
            }
            
            requestS3.Abort();

            if (Fail)
            {
                Done = false;
            }
            else
            {
                Done = true;
            }
        }
        
    }

    void Start()
    {
        // TODO: Point this to your AWS S3 bucket's location
        awsURLBaseVirtual = "https://" +
           awsBucketName +
           ".s3.us-west-2.amazonaws.com/";

        _uploads = new Queue<Upload>();
    }

    public void UploadFile(string fileName, string filePath)
    {
        if (disableUploads)
        {
            return;
        }
        
        var upload = new Upload
        {
            FileName = fileName,
            FilePath = filePath
        };
        _uploads.Enqueue(upload);
    }

    private void Update()
    {
        if (_uploads.Count > 0)
        {
            Uploading = true;
            var latestUpload = _uploads.Peek();
            if (!latestUpload.Done && latestUpload.Thread == null)
            {
                Debug.Log("Starting next upload in queue.");
                latestUpload.StartUpload();
            }

            if (latestUpload.Done)
            {
                Debug.Log("Upload done.");
                _uploads.Dequeue();
            }

            if (latestUpload.Fail)
            {
                Debug.Log("Upload failed, re-enqueueing");
                var oldUpload = _uploads.Dequeue();
                var newUpload = new Upload
                {
                    FileName = oldUpload.FileName,
                    FilePath = oldUpload.FilePath
                };
                _uploads.Enqueue(newUpload);
            }
        }
        else
        {
            Uploading = false;
        }
    }

    // Blocking call
    private bool UploadFileToAWS3(string fileName, string filePath)
    {
        string currentAWS3Date =  
            System.DateTime.UtcNow.ToString(
                "ddd, dd MMM yyyy HH:mm:ss ") +
                "GMT";
        string canonicalString =
            "PUT\n\n\n\nx-amz-date:" +
            currentAWS3Date + "\n/" +
            awsBucketName + "/" + fileName;
        
        UTF8Encoding encode = new UTF8Encoding();
        HMACSHA1 signature = new HMACSHA1();
        
        signature.Key = encode.GetBytes(awsSecretKey);
        byte[] bytes = encode.GetBytes(canonicalString);
        byte[] moreBytes = signature.ComputeHash(bytes);
        string encodedCanonical = Convert.ToBase64String(moreBytes);
        string aws3Header = "AWS " +
            awsAccessKey + ":" +
            encodedCanonical;
        string URL3 = awsURLBaseVirtual + fileName;
        
        WebRequest requestS3 = 
           (HttpWebRequest)WebRequest.Create(URL3);
        
        requestS3.Headers.Add("Authorization", aws3Header);
        requestS3.Headers.Add("x-amz-date", currentAWS3Date);
        
        byte[] fileRawBytes = File.ReadAllBytes(filePath);
        
        requestS3.ContentLength = fileRawBytes.Length;
        requestS3.Method = "PUT";

        try
        {
            Stream s3Stream = requestS3.GetRequestStream();
            s3Stream.Write(fileRawBytes, 0, fileRawBytes.Length);
            Debug.Log("Sent bytes: " +
                      requestS3.ContentLength +
                      ", for file: " +
                      fileName);
            s3Stream.Close();
        }
        catch (Exception e)
        {
            return false;
        }

        return true;
    }
}

Usage:

1. Set up your IAM user and generate the token/secret.
2. Attach this script to a GameObject.
3. Call the UploadFile method with the full file path and filename.