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WebClient client = new WebClient();
client.UploadData("http://localhost/util/SaveAttachment.aspx?id=30&filename=test.doc",fileBuffer);
where fileBuffer is the raw bytes of the file. This page requires edit access so you have to configure authentication the same way as for web services. The file in this example is stored in "/upload/3457/test.doc" or whatever the name of page folder is.