In almost 90% of projects, we need to upload images to server and store them. In most cases, hackers try to exploit an image upload system and try to upload exploitable materials like webshells, some harmful scripts, table deletions scripts, etc.

To prevent this, I have written one helper function which validates file in many conditions and makes sure the file is in correct image format. The code is not fully written by me, I researched many articles and filtered the conditions which helps us to validate the required output.
        /// <summary>
        /// Verifies that a uploading file is in valid Image format
        /// </summary>
        /// <author>
        /// Mayur Lohite
        /// </author>
        /// <param name="postedFile">File which is selected for upload</param>
        /// <param name="imageMinBytes">Minimum file size in byte</param>
        /// <param name="imageMaxBytes">Maximum file size in byte</param>
        /// <returns>true if the file is a valid image format and false if it's not</returns>
        public static bool IsValidImageFormat(HttpPostedFileBase postedFile, int imageMinBytes, long imageMaxBytes)
        {

            //-------------------------------------------
            //  Check the image extension
            //-------------------------------------------
            if (Path.GetExtension(postedFile.FileName).ToLower() != ".jpg"
                && Path.GetExtension(postedFile.FileName).ToLower() != ".png"
                && Path.GetExtension(postedFile.FileName).ToLower() != ".gif"
                && Path.GetExtension(postedFile.FileName).ToLower() != ".jpeg")
            {
                return false;
            }

            //-------------------------------------------
            //  Check the image MIME types
            //-------------------------------------------
            if (postedFile.ContentType.ToLower() != "image/jpg" &&
                        postedFile.ContentType.ToLower() != "image/jpeg" &&
                        postedFile.ContentType.ToLower() != "image/pjpeg" &&
                        postedFile.ContentType.ToLower() != "image/gif" &&
                        postedFile.ContentType.ToLower() != "image/x-png" &&
                        postedFile.ContentType.ToLower() != "image/png")
            {
                return false;
            }



            //-------------------------------------------
            //  Attempt to read the file and check the first bytes
            //-------------------------------------------
            try
            {
                if (!postedFile.InputStream.CanRead)
                {
                    return false;
                }

                if (postedFile.ContentLength < imageMinBytes)
                {
                    return false;
                }

                if (postedFile.ContentLength < imageMinBytes)
                {
                    return false;
                }

                byte[] buffer = new byte[512];
                postedFile.InputStream.Read(buffer, 0, 512);
                string content = System.Text.Encoding.UTF8.GetString(buffer);
                if (Regex.IsMatch(content, @"<script|<html|<head|<title|<body|<pre|<table|<a\s+href|<img|<plaintext|<cross\-domain\-policy",
                    RegexOptions.IgnoreCase | RegexOptions.CultureInvariant | RegexOptions.Multiline))
                {
                    return false;
                }
            }
            catch (Exception)
            {
                return false;
            }

            //-------------------------------------------
            //  Try to instantiate new Bitmap, if .NET will throw exception
            //  we can assume that it's not a valid image
            //-------------------------------------------

            try
            {
                using (var bitmap = new System.Drawing.Bitmap(postedFile.InputStream))
                {
                }
            }
            catch (Exception)
            {
                return false;
            }

            return true;
        }


Hope it will help you. Let me know your thoughts!