fix(file-transfer): fix return type of upload method
fix(file-transfer): fix return type of upload method

fixes #1002

driftyco/ionic-native,xueron/ionic-native,driftyco/ionic-native,xueron/ionic-native,klirix/ionic-native,klirix/ionic-native,driftyco/ionic-native,xueron/ionic-native,klirix/ionic-native,xueron/ionic-native