using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ImageWriter.Helper
{
    public class WriterHelper
    {
        public enum ImageFormat
        {
            bmp,
            jpeg,
            gif,
            tiff,
            png,
            unknown
        }

        public static ImageFormat GetImageFormat(byte[] bytes)
        {
            var bmp = Encoding.ASCII.GetBytes("BM");     // BMP
            var gif = Encoding.ASCII.GetBytes("GIF");    // GIF
            var png = new byte[] { 137, 80, 78, 71 };              // PNG
            var tiff = new byte[] { 73, 73, 42 };                  // TIFF
            var tiff2 = new byte[] { 77, 77, 42 };                 // TIFF
            var jpeg = new byte[] { 255, 216, 255, 224 };          // jpeg
            var jpeg2 = new byte[] { 255, 216, 255, 225 };         // jpeg canon

            if (bmp.SequenceEqual(bytes.Take(bmp.Length)))
                return ImageFormat.bmp;

            if (gif.SequenceEqual(bytes.Take(gif.Length)))
                return ImageFormat.gif;

            if (png.SequenceEqual(bytes.Take(png.Length)))
                return ImageFormat.png;

            if (tiff.SequenceEqual(bytes.Take(tiff.Length)))
                return ImageFormat.tiff;

            if (tiff2.SequenceEqual(bytes.Take(tiff2.Length)))
                return ImageFormat.tiff;

            if (jpeg.SequenceEqual(bytes.Take(jpeg.Length)))
                return ImageFormat.jpeg;

            if (jpeg2.SequenceEqual(bytes.Take(jpeg2.Length)))
                return ImageFormat.jpeg;

            return ImageFormat.unknown;
        }
    }
}

using Microsoft.AspNetCore.Http;
using System.Threading.Tasks;

namespace ImageWriter.Interface
{
    public interface IImageWriter
    {
        Task<string> UploadImage(IFormFile file);
    }
}

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading.Tasks;
using ImageWriter.Helper;
using ImageWriter.Interface;
using Microsoft.AspNetCore.Http;

namespace ImageWriter.Classes
{
    public class ImageWriter : IImageWriter
    {
        public async Task<string> UploadImage(IFormFile file)
        {
            if (CheckIfImageFile(file))
            {
                return await WriteFile(file);
            }

            return "Invalid image file";
        }

        /// <summary>
        /// Method to check if file is image file
        /// </summary>
        /// <param name="file"></param>
        /// <returns></returns>
        private bool CheckIfImageFile(IFormFile file)
        {
            byte[] fileBytes;
            using (var ms = new MemoryStream())
            {
                file.CopyTo(ms);
                fileBytes = ms.ToArray();
            }

            return WriterHelper.GetImageFormat(fileBytes) != WriterHelper.ImageFormat.unknown;
        }

        /// <summary>
        /// Method to write file onto the disk
        /// </summary>
        /// <param name="file"></param>
        /// <returns></returns>
        public async Task<string> WriteFile(IFormFile file)
        {
            string fileName;
            try
            {
                var extension = "." + file.FileName.Split(‘.‘)[file.FileName.Split(‘.‘).Length - 1];
                fileName = Guid.NewGuid().ToString() + extension; //Create a new Name 
                                                              //for the file due to security reasons.
                var path = Path.Combine(Directory.GetCurrentDirectory(), "wwwroot\\images", fileName);

                using (var bits = new FileStream(path, FileMode.Create))
                {
                    await file.CopyToAsync(bits);
                }
            }
            catch (Exception e)
            {
                return e.Message;
            }

            return fileName;
        }
    }
}

using System.Threading.Tasks;
using ImageWriter.Interface;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;

namespace ImageUploader.Handler
{
    public interface IImageHandler
    {
        Task<IActionResult> UploadImage(IFormFile file);
    }

    public class ImageHandler : IImageHandler
    {
        private readonly IImageWriter _imageWriter;
        public ImageHandler(IImageWriter imageWriter)
        {
            _imageWriter = imageWriter;
        }

        public async Task<IActionResult> UploadImage(IFormFile file)
        {
            var result = await _imageWriter.UploadImage(file);
            return new ObjectResult(result);
        }
    }
}

using System.Threading.Tasks;
using ImageUploader.Handler;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;

namespace ImageUploader.Controllers
{
    [Route("api/image")]
    public class ImagesController : Controller
    {
        private readonly IImageHandler _imageHandler;

        public ImagesController(IImageHandler imageHandler)
        {
            _imageHandler = imageHandler;
        }

        /// <summary>
        /// Uplaods an image to the server.
        /// </summary>
        /// <param name="file"></param>
        /// <returns></returns>
        public async Task<IActionResult> UploadImage(IFormFile file)
        {
            return await _imageHandler.UploadImage(file);
        }
    }
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseHsts();
            }

            //Use this to set path of files outside the wwwroot folder
            //app.UseStaticFiles(new StaticFileOptions
            //{
            //    FileProvider = new PhysicalFileProvider(
            //        Path.Combine(Directory.GetCurrentDirectory(), "StaticFiles")),
            //    RequestPath = "/StaticFiles"
            //});

            app.UseStaticFiles(); //letting the application know that we need access to wwwroot folder.

            app.UseHttpsRedirection();
            app.UseMvc();
        }
// This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

            services.AddTransient<IImageHandler, ImageHandler>();
            services.AddTransient<ImageWriter.Interface.IImageWriter, 
                                  ImageWriter.Classes.ImageWriter>();
        }