There is no specific way. Files are just files to the system it does not care if it is a image, video or a dll.
To read a file into a byte stream you can use:
File.ReadAllBytes(fileName);
I personally would use EasyHTTP (https://github.com/hhariri/EasyHttp) to post the file stream:
HttpClient client = new HttpClient();
client.Post(URL, ByteStream, ContentType);