Don't assign text grammar by default. Just use the null grammar.

src/app/text-mate-grammar.coffee

@@ -43,12 +43,10 @@ class TextMateGrammar
     contents = fsUtils.read(path) if not contents? and fsUtils.isFile(path)
 
     if syntax.grammarOverrideForPath(path) is @scopeName
-      Infinity
-    else if @matchesContents(contents)
       3
-    else if @matchesPath(path)
+    else if @matchesContents(contents)
       2
-    else if @isTextGrammar()
+    else if @matchesPath(path)
       1
     else
       -1
@@ -78,9 +76,6 @@ class TextMateGrammar
       pathSuffix = pathComponents[-fileTypeComponents.length..-1]
       _.isEqual(pathSuffix, fileTypeComponents)
 
-  isTextGrammar: ->
-    @scopeName is 'text.plain'
-
   tokenizeLine: (line, ruleStack=[@initialRule], firstLine=false) ->
     originalRuleStack = ruleStack
     ruleStack = new Array(ruleStack...) # clone ruleStack