How to Upload a file using Ajax and a Webservice method

Hi guys,
i need to upload an array of file using a webservice method. I setted up all the things i need to use and the webmethod is ready to accept an array of bytes as a parameter.
How can call that method from Javascript?
this is the webmethod prototype (i'm working on ASPX C#.NET 2.0)
 
 
[WebMethod()]
public string UploadFile(int fieID, string fileName, byte[] file)
{
\\...
}   ...