Upload multipart to AWS S3 Object in .NET Core 3.0

AwsS3Helper.cs

public static class AwsS3Helper
{
	public static async Task UploadObjectStreamAsync(
		this IAmazonS3 s3Client,
		Stream stream,
		UploadStreamToS3Request request)
	{
		async IAsyncEnumerable<UploadPartResponse> UploadAsync(string uploadId)
		{
			await foreach (var (partStream, index, isLast) in SplitPartsFromStream(stream, request.ParSize))
			{
				var partRequest = new UploadPartRequest
				{
					BucketName = request.Bucket,
					Key = request.Key,
					UploadId = uploadId,
					InputStream = partStream,
					PartNumber = index,
					IsLastPart = isLast
				};
				yield return await s3Client.UploadPartAsync(partRequest);
				await partStream.DisposeAsync();
			}
		}

		var initRequest = new InitiateMultipartUploadRequest
		{
			BucketName = request.Bucket,
			Key = request.Key
		};

		if (!string.IsNullOrEmpty(request.DownloadFileName))
			initRequest.Headers.ContentDisposition = $"attachment; filename={request.DownloadFileName}";

		if (!string.IsNullOrEmpty(request.ContentType))
			initRequest.Headers.ContentType = request.ContentType;

		var initResponse = await s3Client.InitiateMultipartUploadAsync(initRequest);

		if (request.ParSize < 1024 * 1024 * 5)
			throw new ArgumentException($"{nameof(request.ParSize)} must be greater than 5MB");

		var uploadAsync = await UploadAsync(initResponse.UploadId).ToListAsync();
		var uploadRequest = new CompleteMultipartUploadRequest
		{
			BucketName = request.Bucket,
			Key = request.Key,
			UploadId = initResponse.UploadId
		};
		uploadRequest.AddPartETags(uploadAsync);

		await s3Client.CompleteMultipartUploadAsync(uploadRequest);
	}

	private static async IAsyncEnumerable<(MemoryStream stream, int index, bool isLast)> SplitPartsFromStream(
		Stream inputStream,
		int partSize)
	{
		inputStream.Position = 0;
		var index = 1;
		for (var remainingBytes = inputStream.Length; remainingBytes > 0; remainingBytes -= partSize)
		{
			var isLast = remainingBytes < partSize;
			var buffer = isLast
				? new byte[remainingBytes]
				: new byte[partSize];

			await inputStream.ReadAsync(buffer, 0, buffer.Length);
			yield return (new MemoryStream(buffer), index, isLast);
			index++;
		}
	}

	private static async Task<IList<T>> ToListAsync<T>(this IAsyncEnumerable<T> asyncEnumerable)
	{
		var list = new List<T>();
		await foreach (var item in asyncEnumerable) list.Add(item);
		return list;
	}
}

public class UploadStreamToS3Request
{
	public string Bucket { get; set; }

	public string Key { get; set; }

	public int ParSize { get; set; } = 1024 * 1024 * 5;

	public string DownloadFileName { get; set; }

	public string ContentType { get; set; }
}