Adding property for downloaded bytes per file.

lib/file.js

@@ -38,6 +38,24 @@ function File (torrent, file) {
   }
 }
 
+Object.defineProperty(File.prototype, 'downloaded', {
+  get: function () {
+    if (!this._torrent.bitfield) return 0
+    var downloaded = 0
+    for (var index = this._startPiece; index <= this._endPiece; ++index) {
+      if (this._torrent.bitfield.get(index)) {
+        // verified data
+        downloaded += this._torrent.pieceLength
+      } else {
+        // "in progress" data
+        var piece = this._torrent.pieces[index]
+        downloaded += (piece.length - piece.missing)
+      }
+    }
+    return downloaded
+  }
+})
+
 File.prototype.select = function (priority) {
   if (this.length === 0) return
   this._torrent.select(this._startPiece, this._endPiece, priority)