support batch getFileSize calls

www/common/pinpad.js

@@ -93,6 +93,21 @@ define([
                 });
             };
 
+            // take a list of channels and return a dictionary of their sizes
+            exp.getMultipleFileSize = function (files, cb) {
+                if (!Array.isArray(files)) {
+                    return window.setTimeout(function () {
+                        cb('[TypeError] pin expects an array');
+                    });
+                }
+                rpc.send('GET_MULTIPLE_FILE_SIZE', files, function (e, res) {
+                    if (e) { return void cb(e); }
+                    if (typeof(res) !== 'object') {
+                        return void cb('INVALID_RESPONSE');
+                    }
+                })
+            };
+
             // get the combined size of all channels (in bytes) for all the
             // channels which the server has pinned for your publicKey
             exp.getFileListSize = function (cb) {