In Headless mode it is not possible to add attachments from a dialog.

Hi,

> because in a headless test that interface is not visible for TC to click on.

UI does not exist in headless mode.

You have just two options possible:

-- Change your test code so it does not need UI for the action (like inclusion of login/password into URL to avoid authorization dialog for Selenium);

-- Do not use headless mode.

Sorry, I just assumed you knew about DOM elements and API's.

See Sending and Receiving HTTP Requests From Script Tests

Here's a small example of POST'ing a JSON message to the URL. 

function httpPostRequest()
{
    var requestBody = JSON.stringify({
        title: 'SmartBear',
        body: 'TestComplete',
        userId: 1,
    });
    
    var aqHttpRequest = aqHttp.CreatePostRequest("https://jsonplaceholder.typicode.com/posts");
    
    aqHttpRequest.SetHeader("Content-Type", "application/json");
    aqHttpRequest.SetHeader("Accept", "application/json;charset=utf-8");
    aqHttpRequest.SetHeader("Accept-Charset", "utf-8");
    
    var aqHttpResponse = aqHttpRequest.Send(requestBody);
    
    Log.Message(aqHttpResponse.StatusCode);
    Log.Message(aqHttpResponse.Text);
}

You can do something similar, providing your website doesn't require authentication, that is when it gets tricky. You need to construct the POST Header, you can copy the information that you see in DevTools; you then need to attach your file in binary format (I think it's Base64 see post https://community.smartbear.com/t5/TestComplete-Questions/Encrypt-Decrypt-Base64-string-in-TestComplete/td-p/235262). Hopefully, you should be able to upload your file 🤔. You might require help from your developers!

I don't have dotNET package installed in TC, so I can't provide an example.