Always FileNotFoundException Permission Denied in android

Why do i always get this error FileNotFoundException Permission Denied? The code is going smooth but when i click a file to download, it wont be downloaded. Please help me. im new to this

Here is my logcat

03-28 09:19:34.695: E/log_tag(17921): eer java.io.FileNotFoundException: /mnt/sdcard/Excel.xlsx (Permission denied)

In my manifest

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>