I think you are trying to access applicationDirectory. In iOS standard directories native path like
app:/ means applicationDirectory
applicationDirectory - /var/mobile/Applications/uid/filename.app
applicationStorageDirectory - /var/mobile/Applications/uid/Library/Application Support/applicationID/Local Store
However you can access application directory file like
var applicationXMLFile:File = File.applicationDirectory.resolvePath("application_data.xml");
var applicationXMLUpdateFile:File = new File(applicationXMLFile.nativePath);
if we are trying access directly like
var applicationXMLFile:File = File.applicationDirectory.resolvePath("application_data.xml");
Sometimes it throws SecurityError/IOError
.