add fxA ui elements

app/archive.js

@@ -1,3 +1,4 @@
+/* global MAXFILESIZE */
 import { blobStream, concatStream } from './streams';
 
 export default class Archive {
@@ -34,4 +35,21 @@ export default class Archive {
   get stream() {
     return concatStream(this.files.map(file => blobStream(file)));
   }
+
+  addFiles(files) {
+    const newSize = files.reduce((total, file) => total + file.size, 0);
+    if (this.size + newSize > MAXFILESIZE) {
+      return false;
+    }
+    this.files = this.files.concat(files);
+    return true;
+  }
+
+  checkSize() {
+    return this.size <= MAXFILESIZE;
+  }
+
+  remove(index) {
+    this.files.splice(index, 1);
+  }
 }