If you're used to working with Django, you've probably done so before on a single host with its own file system. For example, media files will probably be stored in a directory in the root of the project directory.
This gives you, and Python, direct access to those files.
No persistent local file storage
The Divio Cloud architecture works a little differently. The same site may be running as several different instances, on several different hosts (this is one way we're able to scale so hugely - we can simply create new instances to handle increasing demand).
Although each of those instances will indeed have its own local file storage, it will be independent of each of the others, and it won't persist - once that instance ceases to exist, so will the files.
This means a project can't upload files, save them to disk, and then expect to find them again. They might be there, if you happen to land on the same instance when asking for them, but they might equally well not be.
Amazon S3 storage
Instead, we use Amazon S3 for file storage. It's an independent persistent storage system, that all instances of a particular site will equal and transparent access to.
Use Django's storage API
For most Django applications, this won't be an issue, nor will you need to learn about S3.
Django is able to use multiple storage backends, all addressed through a common API. This is the safe and correct way to handle files in Django, so that applications can abstract from details of the storage implementation, and simply not need to know about it.
As long as the application uses Django's storage API, rather than attempting to manipulate Python File objects directly, it doesn't need to do anything differently.
Similarly, an application should not rely on knowing or manipulating a File object's file path.
If you are using Django Filer, please use the Django Filer Addon from the Marketplace, rather than installing the Filer into your project via its requirements. This is because Filer currently requires its
THUMBNAIL_DEFAULT_STORAGE to be set manually. See the Django Filer Addon code to see how we do this.