First create some placeholder for binary extension :
const string BINARY_EXTENSION = "bin";
In btnSaveFile_Click()
You can modify the save functionality with:
if ( filename.EndsWith(BINARY_EXTENSION))
File.WriteAllBytes(filename, Encoding.UTF8.GetBytes(tbEditor.Text)); // Or choose something different then UTF8
else
File.WriteAllText(filename);
And inside your btnOpenFile_Click
you can do the same :
if ( filename.EndsWith(BINARY_EXTENSION))
tbEditor.Text = Encoding.UTF8.GetString(File.ReadAllBytes(filename); // Or choose something different then UTF8
else
tbEditor.Text = File.ReadAllText(filename);