While using APK Expansion files in Android unlike earlier new android google play developer console in Account details
erlier per android google play developer console Account there is one public key, now per each app on the google play developer console Account has the unique public key.
for testin apk expansion file upload the app, after uploadin the apk you see some options like this
click on service & API
select the public key copy paste in your sample
public class SampleDownloaderService extends DownloaderService {
// stuff for LVL -- MODIFY FOR YOUR APPLICATION!
private static final String BASE64_PUBLIC_KEY ="paste here";
while uploading apk you should upload any file (may be zipped file) with any name.. you need to make sure of the size of the file you are uploading is the file size you have put in the sampledownloader Activity
private static final XAPKFile[] xAPKS = { new XAPKFile(true, 1, 425553759L)
1 is the android:versionCode specified in the manifest.
what you uploaded xapk file while uploading apk same file will be renamed and downloaded as obb file in sd card
For unziping the obb file and more in click here