Attaching files to Azure DevOps work item - api

Question

I am trying to attach files (screenshots) to an Azure DevOps work item via a C# desktop app. I have managed to attach files, but they're not valid image files, which leads me to believe that I'm doing something wrong in uploading them.
From the documentation DevOps Create Attachment below is the section on the Request body of the API call, which is rather vague.
From a GitHub discussion this answer seems to suggest that I just upload the binary content directly, which is what I'm doing.
My code is as follows
var img = File.ReadAllBytes(fname);
string query = #"/_apis/wit/attachments?fileName=" + fname + #"&api-version=6.0"
string response = AzureUtils.AttachFile(query, img, "POST", false, "application/octet-stream");
Is it correct that I literally pass in the byte array which is read from the file (variable img) as the body?
Why is it not a valid file when I look at it in DevOps?
The code for AttachFile is
public static string AttachFile(string query, byte[] data = null, string method = "GET",
bool dontUseBaseURL = false, string contentType = "application/json-patch+json")
{
try
{
HttpWebRequest request = WebRequest.Create(query) as HttpWebRequest;
request.ContentType = contentType;
request.Method = method;
request.Proxy.Credentials = CredentialCache.DefaultCredentials;
request.Headers.Add("Authorization", "Basic " +
Convert.ToBase64String(Encoding.ASCII.GetBytes(string.Format("{1}", ["AzurePAT"]))));
if (data != null)
{
using (StreamWriter writer = new StreamWriter(request.GetRequestStream()))
{
writer.Write(data);
}
}
HttpWebResponse response = request.GetResponse() as HttpWebResponse;
string result = string.Empty;
using (StreamReader reader = new StreamReader(response.GetResponseStream()))
{
result = reader.ReadToEnd();
}
request = null;
response = null;
return result;
}