AFAIK You need to set WRITE_EXTERNAL_STORAGE permission to your applications XML descriptor file (yourAppName-app.xml)
<android>
<manifestAdditions><![CDATA[
<manifest>
<!-- See the Adobe AIR documentation for more information about setting Google Android permissions -->
...
...
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
...
...
</manifest>
]]></manifestAdditions>
</android>