Move decryptFile into a utility function so that it can be shared between different components

Conflicts:
	src/components/views/messages/MImageBody.js

src/utils/DecryptFile.js

@@ -0,0 +1,42 @@
+/*
+Copyright 2016 OpenMarket Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+'use struct';
+
+// Pull in the encryption lib so that we can decrypt attachments.
+var encrypt = require("browser-encrypt-attachment");
+// Pull in a fetch polyfill so we can download encrypted attachments.
+require("isomorphic-fetch");
+// Grab the client so that we can turn mxc:// URLs into https:// URLS.
+var MatrixClientPeg = require('../MatrixClientPeg');
+
+
+export function decryptFile(file) {
+    var url = MatrixClientPeg.get().mxcUrlToHttp(file.url);
+    var self = this;
+    // Download the encrypted file as an array buffer.
+    return fetch(url).then(function (response) {
+        return response.arrayBuffer();
+    }).then(function (responseData) {
+        // Decrypt the array buffer using the information taken from
+        // the event content.
+        return encrypt.decryptAttachment(responseData, file);
+    }).then(function(dataArray) {
+        // Turn the array into a Blob and give it the correct MIME-type.
+        var blob = new Blob([dataArray], {type: file.mimetype});
+        return blob;
+    });
+}