Add shared getFileExtension helper method

Fix
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12198#note_37146854

Conflicts:
	app/assets/javascripts/repo/helpers/repo_helper.js

app/assets/javascripts/repo/helpers/repo_helper.js

@@ -40,7 +40,7 @@ const RepoHelper = {
   },
 
   getLanguageIDForFile(file, langs) {
-    const ext = file.name.split('.').pop();
+    const ext = RepoHelper.getFileExtension(file.name);
     const foundLang = RepoHelper.findLanguage(ext, langs);
 
     return foundLang ? foundLang.id : 'plaintext';

app/assets/javascripts/repo/services/repo_service.js

@@ -2,6 +2,7 @@
 import axios from 'axios';
 import Store from '../stores/repo_store';
 import Api from '../../api';
+import Helper from '../helpers/repo_helper';
 
 const RepoService = {
   url: '',
@@ -29,7 +30,7 @@ const RepoService = {
   },
 
   urlIsRichBlob(url = this.url) {
-    const extension = url.split('.').pop();
+    const extension = Helper.getFileExtension(url);
 
     return this.richExtensionRegExp.test(extension);
   },