PLT-3487 Preview uploaded txt file (#4145)

* fix filename

* Add 'txt' to hilighted language

webapp/components/code_preview.jsx

@@ -4,7 +4,7 @@
 import $ from 'jquery';
 import React from 'react';
 
-import * as SyntaxHighlighting from 'utils/syntax_hightlighting.jsx';
+import * as SyntaxHighlighting from 'utils/syntax_highlighting.jsx';
 import Constants from 'utils/constants.jsx';
 
 import FileInfoPreview from './file_info_preview.jsx';