A modified version of the FreshDesk C# API sample for creating a ticket with an attachment: https://github.com/freshdesk/fresh-samples/blob/master/C-Sharp/CreateTicketWithAttachment.cs

FreshDesk.cs

/*----------------------------------------------------------------------------------------------------------
 *  THIS CLASS DEPENDS ON THE FOLLOWING:
 *      System.Web.Extensions reference
 *      MimeTypeMap.List nuget package (https://www.nuget.org/packages/MimeTypeMap.List/)
 *
 *
 *  ALSO: do not forget to fill in your FreshDesk domain and FreshDesk API (lines 23 & 24)
 * ----------------------------------------------------------------------------------------------------------
 */
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;
using System.Web.Script.Serialization;

namespace Freshdesk
{
    public class FreshDesk
    {
        private const string fdDomain = "YOUR_DOMAIN";
        private const string _APIKey = "YOUR_API_KEY";
        private const string path = "/api/v2/tickets";
        private const string _Url = "https://" + fdDomain + ".freshdesk.com" + path;

        public static string SubmitTicket(string name, string email, string subject, string description, List<string> attachments = null)
        {
            Dictionary<string, string> payload = new Dictionary<string, string>
            {
                { "name", name },
                { "email", email },
                { "subject", subject },
                { "description", description },
                { "status", "2" },
                { "priority", "2" },
            };

            string timestamp = $"{DateTime.Now.Ticks:x}";
            string boundary = $"----------------------------{timestamp}";

            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(_Url);
            request.Headers.Clear();
            request.ContentType = $"multipart/form-data; boundary={boundary}";
            request.Method = "POST";
            request.KeepAlive = true;

            string login = $"{_APIKey}:X"; // It could be your username:password also.
            string credentials = Convert.ToBase64String(Encoding.UTF8.GetBytes(login));
            request.Headers[HttpRequestHeader.Authorization] = $"Basic {credentials}";

            using (Stream requestStream = request.GetRequestStream())
            {
                foreach (string key in payload.Keys)
                {
                    byte[] data = Encoding.UTF8.GetBytes($"--{boundary}\r\nContent-Disposition: form-data; name=\"{key}\"\r\n\r\n{payload[key]}\r\n");
                    requestStream.Write(data, 0, data.Length);
                }

                if (attachments != null)
                {
                    foreach (string filePath in attachments)
                    {
                        FileInfo file = new FileInfo(filePath);
                        string contentType = MimeTypeMap.List.MimeTypeMap.GetMimeType(file.Extension).First();

                        byte[] attachment = Encoding.UTF8.GetBytes($"--{boundary}\r\nContent-Disposition: form-data; name=\"attachments[]\"; filename=\"{file.Name}\"\r\n" +
                                                                    $"Content-Type: {contentType}\r\n\r\n");
                        using (FileStream fileStream = new FileStream(file.FullName, FileMode.Open, FileAccess.Read))
                        {
                            byte[] fileData = new byte[fileStream.Length];
                            fileData.Concat(new byte[fileStream.Length]);
                            fileStream.Read(fileData, 0, fileData.Length);

                            attachment = attachment.Concat(fileData).Concat(Encoding.UTF8.GetBytes("\r\n")).ToArray();
                        }

                        requestStream.Write(attachment, 0, attachment.Length);
                    }
                }

                byte[] finalBoundaryBytes = Encoding.UTF8.GetBytes($"--{boundary}--");
                requestStream.Write(finalBoundaryBytes, 0, finalBoundaryBytes.Length);
            }

            try
            {
                Console.WriteLine("Submitting Request");
                
                using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
                {
                    using (Stream dataStream = response.GetResponseStream())
                    {
                        using (StreamReader reader = new StreamReader(dataStream, Encoding.UTF8))
                        {
                            string responseBody = reader.ReadToEnd();
                            JavaScriptSerializer serializer = new JavaScriptSerializer();
                            Dictionary<string, object> json = (Dictionary<string, object>)serializer.DeserializeObject(responseBody);
                            
                            //return status code
                            Console.WriteLine($"Status Code: {(int)response.StatusCode} {response.StatusCode}");
                            //return location header
                            Console.WriteLine($"Location: {response.Headers["Location"]}");
                            //return the response 
                            Console.Out.WriteLine(responseBody);

                            System.Diagnostics.Debug.WriteLine($"X-Request-Id: {response.Headers["X-Request-Id"]}");
                            System.Diagnostics.Debug.WriteLine($"Timestamp: {DateTime.UtcNow}");

                            return $"https://{fdDomain}.freshdesk.com/a/tickets/{json["id"]}"; //Return the ticket URL
                        }
                    }
                }
            }
            catch (WebException ex)
            {
                Console.WriteLine($"Failed to submit FreshDesk ticket ({((HttpWebResponse)ex.Response).StatusCode} {(int)((HttpWebResponse)ex.Response).StatusCode}): {ex.Message} {ex.StackTrace}");
                Console.WriteLine($"X-Request-Id: {ex.Response.Headers["X-Request-Id"]} ; Timestamp: {timestamp}");
                using (Stream responseStream = ex.Response.GetResponseStream())
                {
                    using (StreamReader reader = new StreamReader(responseStream))
                    {
                        Logger.LogError(reader.ReadToEnd());
                    }
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("ERROR");
                Console.WriteLine(ex.Message);
            }
            
            return null; //Return a null URL in case of error
        }
    }
}