only check for shebangs in executable files

lib/linguist/blob_helper.rb

@@ -146,7 +146,8 @@ module Linguist
     #
     # Return true or false
     def shebang_extname?
-      extname.empty?
+      extname.empty? &&
+        (mode.to_i(8) & 05) == 05
     end
 
     MEGABYTE = 1024 * 1024

lib/linguist/file_blob.rb

@@ -32,6 +32,13 @@ module Linguist
     # Returns a String
     attr_reader :name
 
+    # Public: Read file permissions
+    #
+    # Returns a String like '100644'
+    def mode
+      File.stat(@path).mode.to_s(8)
+    end
+
     # Public: Read file contents.
     #
     # Returns a String.