add description for text/plain

src/main/java/eu/siacs/conversations/utils/UIHelper.java

@@ -502,6 +502,8 @@ public class UIHelper {
             return context.getString(R.string.ebook);
         } else if (mime.equals("application/gpx+xml")) {
             return context.getString(R.string.gpx_track);
+        } else if (mime.equals("text/plain")) {
+            return context.getString(R.string.plain_text_document);
         } else {
             return mime;
         }

src/main/res/values/strings.xml

@@ -965,4 +965,5 @@
     <string name="no_active_accounts_support_this">No active accounts support this feature</string>
     <string name="backup_started_message">The backup has been started. You’ll get a notification once it has been completed.</string>
     <string name="unable_to_enable_video">Unable to enable video.</string>
+    <string name="plain_text_document">Plain text document</string>
 </resources>