求救：如何过滤FPRPC response

本人在开发sharepoint上过滤文件的下载response，做法是这样的：
在HttpApplication的PreProcessHttpRequest事件里，将Response.Filter替换为自己实现的Stream对象，用来过滤Response里的文件内容。代码如下：

对于GET以及WebDAV的下载文件请求的处理是正常的，但是对于FPRPC的"get document"命令，在DefaultHttpHandler.BeginProcessRequest()函数里就会抛出一个HttpException异常："The Http verb Post used to access Path '/_vti_bin/_vti_aut/author.dll' is not allowed".

请问这到底是什么原因？难道原先的Response.Filter不能被覆盖吗？
希望有专家可以解答，非常感谢！这个事情比较急啊.

public abstract class HttpFilterBase : Stream
    {
        private Stream _baseStream;
        private bool _closed;

        protected Stream BaseStream
        {
            get { return this._baseStream; }
        }

        public override bool CanRead
        {
            get { return false; }
        }

        public override bool CanWrite
        {
            get { return !_closed; }
        }

        public override bool CanSeek
        {
            get { return false; }
        }

        protected bool Closed
        {
            get { return _closed; }
        }

        public override long Length
        {
            get { throw new NotSupportedException(); }
        }

        public override long Position
        {
            get { throw new NotSupportedException(); }
            set { throw new NotSupportedException(); }
        }

        protected HttpFilterBase(Stream _baseStream)
        {
            this._baseStream = _baseStream;
            this._closed = false;
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            _baseStream.Write(buffer, offset, count);
        }

        public override void Close()
        {
            if (!_closed)
            {
                _closed = true;
                _baseStream.Close();
            }
        }

        public override void Flush()
        {
            _baseStream.Flush();
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            throw new NotSupportedException();
        }

        public override long Seek(long offset, SeekOrigin origin)
        {
            throw new NotSupportedException();
        }

        public override void SetLength(long value)
        {
            throw new NotSupportedException();
        }
    }

    // This filter changes all characters passed through it to uppercase.
    public class HEResponseFilterStream : HttpFilterBase
    {
        private HttpApplication application;

        public HEResponseFilterStream(Stream sink, Object source)
            : base(sink)
        {
            application = (HttpApplication)source;
        }

        public override void Close()
        {
            base.Close();
        }

        public override void Flush()
        {
            HttpContext context = application.Context;
            HttpRequest Request = context.Request;
            HttpResponse Response = context.Response;

            base.Flush();
        }

        // The Write method actually does the filtering.
        public override void Write(byte[] buffer, int offset, int count)
        {
            HttpContext context = application.Context;
            HttpRequest Request = context.Request;
            HttpResponse Response = context.Response;

           {
                base.Write(buffer, offset, count);
                return;
            }
        }
    }

补充：.NET技术 ,  ASP.NET