Saving an Image in C# ..Please Help

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C#

Hi Guys.
I have created a simple windows app in c# that enables users to convert images to jpg, png and bmp files.
This works well but my issue is that when ever i press the the "save button" the opens the save dialogue and asks the user to type in the name of the file as they would like it to be saved as ... but instead of the user to type the whole name ..e.g. "button.png" is there a way that i could code this so that the user could only type in "button" and the file automatically saves as "button.png" without the user actually typing in the the .PNG .. I hope this makes sense.
CODE:
i have 3 radio buttons to be selected by the user
radio button 1 = jpeg
radio button 2 = png
radio bbutton 3 = bmp

private void button2_Click(object sender, EventArgs e)
{
    DialogResult saveimage = saveFileDialog1.ShowDialog();

    if (saveimage == DialogResult.OK)
    {

        if (radioButton1.Checked)
        {

            file.Save(saveFileDialog1.FileName, ImageFormat.Jpeg);
        }

        if (radioButton2.Checked)
        {

            file.Save(saveFileDialog1.FileName , ImageFormat.Png);

        }



        if (radioButton3.Checked)
        {

            file.Save(saveFileDialog1.FileName, ImageFormat.Bmp);
        }

    }