load file at RichTextBox

HI , I am new here and I am beginner at C sharp
so my question is:
I want to load file at richtextbox
so I write this code and it worked :

Code:

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DialogResult result = this.openFileDialog1.ShowDialog();
            if (result == DialogResult.OK)
                this.richTextBox1.LoadFile(openFileDialog1.FileName, RichTextBoxStreamType.PlainText);

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but that open just txt files and I want to open all the types of files without make if for each extension , is there anyway for that
thanks ...