getting extension of file

Look at the file stream before you try to use it. .docx files are just renamed .zip files, so I think if the first two bytes of the file stream are PK then you can be fairly certain that someone renamed a .docx file and you can avoid opening it.

it's funny, your problem is in the format of the file, if you're only opening Word 2003 files then your app doesn't know how to handle the 2007 files and renaming it won't change that fact.

I'll show you what I mean, change the extension of the docx file to zip and open it using any application that can read zipped files... now try the same with a doc file... see the problem?

and btw, there's no way to know if the extension of a file was changed previously

some body gave me this link in above discussion
http://www.garykessler.net/library/file_sigs.html

it has solved my problem..
thanks bro.

//http://www.garykessler.net/library/file_sigs.html

            int i = 0;
            int count = 0;
            OpenFileDialog op = new OpenFileDialog();
            if (op.ShowDialog() == DialogResult.OK)
            {
                FileStream fstream = new FileStream(op.FileName, FileMode.Open);
                while ((i = fstream.ReadByte()) != -1)
                {
                    if (count == 10) break;
                    count++;
                    textBox1.Text += i.ToString() + " ";
                }

                //80 75 3 4 20 0 6 0 8 0 is bytecode for docx
                if (textBox1.Text.Equals("80 75 3 4 20 0 6 0 8 0 "))
                {
                    MessageBox.Show("docx file");
                    return;
                }
                MessageBox.Show("Not Docx");
            }

Trial and error method: rename it (with the supposed original extension) and try to open it with an application requiring files with such an extension.
:-)