add rtf and txt as supported file formats

(cherry picked from commit 829b2b77b9)
Signed-off-by: Andras Timar <andras.timar@collabora.com>

js/viewer/viewer.js

@@ -12,6 +12,9 @@ var odfViewer = {
 		'application/vnd.wordperfect',
 		'application/msonenote',
 		'application/msword',
+		'application/rtf',
+		'text/rtf',
+		'text/plain',
 		'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
 		'application/vnd.openxmlformats-officedocument.wordprocessingml.template',
 		'application/vnd.ms-word.document.macroEnabled.12',