There were several errors in the OP code.
Using the snippet provided by @mabu and the code that I found at http://www.briangrinstead.com/blog/multipart-form-post-in-c , here is the * Function Code "Code Block for Download" to the Jira.
public bool AddAttachments(string issueKey, IEnumerable<string> filePaths) { string restUrl = Jira.FormatRestUrl(m_JiraId, true); string issueLinkUrl = String.Format("{0}/issue/{1}/attachments", restUrl, issueKey); var filesToUpload = new List<FileInfo>(); foreach (var filePath in filePaths) { if (!File.Exists(filePath)) { Jira.LogError("File '{0}' doesn't exist", filePath); return false; } var file = new FileInfo(filePath); if (file.Length > 10485760) // TODO Get Actual Limit { Jira.LogError("Attachment too large"); return false; } filesToUpload.Add(file); } if (filesToUpload.Count <= 0) { Jira.LogWarning("No file to Upload"); return false; } return PostMultiPart(issueLinkUrl, filesToUpload); } private Boolean PostMultiPart(string restUrl, IEnumerable<FileInfo> filePaths) { HttpWebResponse response = null; HttpWebRequest request = null; try { var boundary = string.Format("----------{0:N}", Guid.NewGuid()); var content = new MemoryStream(); var writer = new StreamWriter(content); foreach (var filePath in filePaths) { var fs = new FileStream(filePath.FullName, FileMode.Open, FileAccess.Read); var data = new byte[fs.Length]; fs.Read(data, 0, data.Length); fs.Close(); writer.WriteLine("--{0}", boundary); writer.WriteLine("Content-Disposition: form-data; name=\"file\"; filename=\"{0}\"", filePath.Name); writer.WriteLine("Content-Type: application/octet-stream"); writer.WriteLine(); writer.Flush(); content.Write(data, 0, data.Length); writer.WriteLine(); } writer.WriteLine("--" + boundary + "--"); writer.Flush(); content.Seek(0, SeekOrigin.Begin); request = WebRequest.Create(restUrl) as HttpWebRequest; if (request == null) { Jira.LogError("Unable to create REST query: {0}", restUrl); return false; } request.Method = "POST"; request.ContentType = string.Format("multipart/form-data; boundary={0}", boundary); request.Accept = "application/json"; request.Headers.Add("Authorization", "Basic " + m_EncodedCredential); request.Headers.Add("X-Atlassian-Token", "nocheck"); request.ContentLength = content.Length; using (Stream requestStream = request.GetRequestStream()) { content.WriteTo(requestStream); requestStream.Close(); } using (response = request.GetResponse() as HttpWebResponse) { if (response.StatusCode != HttpStatusCode.OK) { var reader = new StreamReader(response.GetResponseStream()); Jira.LogError("The server returned '{0}'\n{1}", response.StatusCode, reader.ReadToEnd()); return false; } return true; } } catch (WebException wex) { if (wex.Response != null) { using (var errorResponse = (HttpWebResponse)wex.Response) { var reader = new StreamReader(errorResponse.GetResponseStream()); Jira.LogError("The server returned '{0}'\n{1}).", errorResponse.StatusCode, reader.ReadToEnd()); } } if (request != null) { request.Abort(); } return false; } finally { if (response != null) { response.Close(); } } }