iOS large file on which directory

My program is an on-demand video, allowing users to download screen, I ask the user to download the screen should be stored in which directory it? Understand what ios sandbox, said he can only operate their own that several directories, document, tmp, library, caches;
That I should let users to download video files by default folder to which it?
Reply:
Is generally placed in the Library / Caches it
Reply:
document: the directory file, the backup will be backed up App; upgrade when you can also reserve;
tmp: system regularly delete;
library: forget their own look.
caches: backup or upgrade, not preserved. The system will not be deleted, you need to manually delete unused files.

Conclusion: Look business requirements.
Reply:
Upstairs very detailed, add this: 5.0 after the system in some special cases may remove content caches directory.
On iOS 5.0 and later, the system may delete the Caches directory on rare occasions when the system is very low on disk space.