conversation: support files drag and drop

Change-Id: If97d5b8602ab162b605b1df02c9528d6609d4f0c
diff --git a/ui/Base.lproj/Localizable.strings b/ui/Base.lproj/Localizable.strings
index 4df82a2..e950731 100644
--- a/ui/Base.lproj/Localizable.strings
+++ b/ui/Base.lproj/Localizable.strings
@@ -271,3 +271,6 @@
 /* conference state tooltip moderator */
 "Moderator" = "Moderator";
 
+/* drop files */
+"Drop files to send" = "Drop files to send";
+