diff --git a/src/calibre/gui2/tweak_book/editor/__init__.py b/src/calibre/gui2/tweak_book/editor/__init__.py
index f1038c15f6..ce73756a3e 100644
--- a/src/calibre/gui2/tweak_book/editor/__init__.py
+++ b/src/calibre/gui2/tweak_book/editor/__init__.py
@@ -11,12 +11,15 @@ from PyQt4.Qt import QTextCharFormat
 from calibre.ebooks.oeb.base import OEB_DOCS, OEB_STYLES
 from calibre.ebooks.oeb.polish.container import guess_type
 
+_xml_types = {'application/oebps-page-map+xml', 'application/vnd.adobe-page-template+xml'} | {
+            guess_type('a.'+x) for x in ('ncx', 'opf', 'svg', 'xpgt', 'xml')}
+
 def syntax_from_mime(name, mime):
     if mime in OEB_DOCS:
         return 'html'
     if mime in OEB_STYLES:
         return 'css'
-    if mime in {guess_type('a.svg'), guess_type('a.opf'), guess_type('a.ncx'), guess_type('a.xml'), 'application/oebps-page-map+xml', 'application/vnd.adobe-page-template+xml'}:
+    if mime in _xml_types:
         return 'xml'
     if mime.startswith('text/'):
         return 'text'