controller.cs

[HttpGet("{bucketName}/{fileName}")]
        public async Task<IActionResult> DownloadFileAsync([FromRoute]string bucketName, [FromRoute]string fileName)
        {
            var objectRequest = await _service.DownloadFileAsync(bucketName, fileName);

            if (objectRequest.Status == System.Net.HttpStatusCode.OK)
            {
                Response.Headers.Add("Content-Length", objectRequest.Content_Length.ToString());
                return new FileStreamResult(objectRequest.FileStream, "application/octet-stream")
                {
                    LastModified = objectRequest.LastModified,
                    FileDownloadName = objectRequest.FileName
                };
            }
            else
            {
                return BadRequest(objectRequest.Message);
            }
        }

S3StreamResponse.cs

public class S3StreamResponse : S3Response
    {
        public Stream FileStream { get; set; }
        public String FileName { get; set; }
        public String Content_Type { get; set; }
        public long Content_Length { get; set; }
        public DateTime LastModified { get; set; }
    }

service.cs

  public async Task<S3StreamResponse> DownloadFileAsync(string bucketName, string fileName)
        {
            try {
                var getRequest = new GetObjectRequest()
                {
                    BucketName =bucketName,
                    Key = fileName
                };

                var response = await _client.GetObjectAsync(getRequest);

                return new S3StreamResponse()
                {
                    Status = System.Net.HttpStatusCode.OK,
                    Content_Length = response.ContentLength,
                    FileStream = response.ResponseStream,
                    LastModified = response.LastModified,
                    FileName = response.Key,
                    Message = "Successfully returned stream body"
                };
            }
            catch (AmazonS3Exception e)
            {
                Debug.WriteLine(e.Message);
                return new S3StreamResponse()
                {
                    Status = System.Net.HttpStatusCode.BadRequest,
                    Message = e.Message.Replace("key", "file")
                };
            }
            catch (Exception e)
            {
                Debug.WriteLine(e.Message);
                return new S3StreamResponse()
                {
                    Status = System.Net.HttpStatusCode.BadRequest,
                    Message = e.Message
                };
            }
        }