FileController.cs

	[Route("api/[controller]")]
	public class FileController : ControllerBase
	{
		[HttpPost("uploadlarge")]
		[DisableFormValueModelBinding]
		public async Task<IActionResult> UploadLargeFileAsync()
		{
			try
			{
				if (!MultipartRequestHelper.IsMultipartContentType(Request.ContentType))
				{
					throw new FormatException("Form without multipart content.");
				}

				var subDirectory = string.Empty;
				var count = 0;
				var totalSize = 0L;
        // find the boundary
				var boundary = MultipartRequestHelper.GetBoundary(MediaTypeHeaderValue.Parse(Request.ContentType));
        // use boundary to iterator through the multipart section
				var reader = new MultipartReader(boundary, HttpContext.Request.Body);

				var section = await reader.ReadNextSectionAsync();
				do
				{
					ContentDispositionHeaderValue.TryParse(section.ContentDisposition, out var contentDisposition);
					if (!MultipartRequestHelper.HasFileContentDisposition(contentDisposition))
					{
						if (MultipartRequestHelper.HasFormDataContentDisposition(contentDisposition) && contentDisposition.Name == nameof(subDirectory))
						{
							using var streamReader = new StreamReader(section.Body, Encoding.UTF8);
              // get the subdirectory first
							subDirectory = streamReader.ReadToEnd();
						}

						section = await reader.ReadNextSectionAsync();
						continue;
					}
					totalSize += await SaveFileAsync(section, subDirectory);

					count++;
					section = await reader.ReadNextSectionAsync();
				} while (section != null);

				return Ok(new { Count = count, Size = SizeConverter(totalSize) });

			}
			catch (Exception exception)
			{
				return BadRequest($"Error: {exception.Message}");
			}
		}

		private async Task<long> SaveFileAsync(MultipartSection section, string subDirectory)
		{
			subDirectory ??= string.Empty;
			var target = Path.Combine("{root}", subDirectory);
			Directory.CreateDirectory(target);

			var fileSection = section.AsFileSection();

			var filePath = Path.Combine(target, fileSection.FileName);
			using var stream = new FileStream(filePath, FileMode.Create, FileAccess.Write, FileShare.None, 1024);
			await fileSection.FileStream.CopyToAsync(stream);

			return fileSection.FileStream.Length;
		}

		private string SizeConverter(long bytes)
		{
			var fileSize = new decimal(bytes);
			var kilobyte = new decimal(1024);
			var megabyte = new decimal(1024 * 1024);
			var gigabyte = new decimal(1024 * 1024 * 1024);

			switch (fileSize)
			{
				case var _ when fileSize < kilobyte:
					return $"Less then 1KB";
				case var _ when fileSize < megabyte:
					return $"{Math.Round(fileSize / kilobyte, 0, MidpointRounding.AwayFromZero):##,###.##}KB";
				case var _ when fileSize < gigabyte:
					return $"{Math.Round(fileSize / megabyte, 2, MidpointRounding.AwayFromZero):##,###.##}MB";
				case var _ when fileSize >= gigabyte:
					return $"{Math.Round(fileSize / gigabyte, 2, MidpointRounding.AwayFromZero):##,###.##}GB";
				default:
					return "n/a";
			}
		}
	}