Syntax Highlight Not Available in Eclipse Plugin

I associated the .ext extension with Oxygen XML Developer plugin in Eclipse. Why does an .ext file opened with the Oxygen XML Developer plugin plugin not have syntax highlight?

Associating an extension with Oxygen XML Developer plugin in Eclipse versions 3.6-3.8, 4.2-4.5 requires three steps:

Associate the .ext extension with the Oxygen XML Developer plugin plugin.
1. Open the Preferences dialog box and go to General > Editors > File Associations.
2. Add *.ext to the list of file types.
3. Select *.ext in the list by clicking it.
4. Add Oxygen XML Developer plugin to the list of Associated editors and make it the default editor.
Associate the .ext extension with the Oxygen XML content type.
1. Open the Preferences dialog box and go to General > Content Types.
2. Add *.ext to the File associations list for the Text > XML > Oxygen XML Developer plugin content type.
Press the OK button in the Eclipse preferences dialog box.

Now when an *.ext file is opened the icon of the editor and the syntax highlight should be the same as for XML files opened with the Oxygen XML Developer plugin plugin.