Hi,
Posting a file is much more complicated than you think. You need to format the request data exactly as it should be and you can't just pull things out of your head and hope it would work. So the first step is to search online to see how the request data should be formatted.
The next step for you is to determine whether you need to do that with EO.WebBrowser or not. You do not need to use EO.WebBrowser to upload a file. EO.WebBrowser is mainly for you to load and run a web page. If you just want to send a file to the server, you can just use .NET's built in WebRequest class.
When you send a request to the server, you will get a response. EO.WebBrowser becomes useful when it comes to the response. For example, if you were to upload a picture and the server sends back a response that contains complex UI that allows you to preview/edit the picture (such as ShutterFly), then in order to present that complex UI to the end user you will need EO.WebBrowser. On the other hand, if you are just interested in a HTTP result code (or some other kind of structural data result such as XML) to indicate whether the upload is successful or not, then you can get those information directly through .NET's built-in support.
If you indeed need to use EO.WebBrowser, you need to follow the steps here to format the request data:
https://www.essentialobjects.com/doc/eo.webbrowser.postdataitemconstructor1.aspxHope this points you to the right direction.
Thanks!