We use an ephemeral file system which means that every time your app is restarted or deployed we use a fresh copy of your application. For this reason all files which have been changed or created by the app are cleaned with every restart and deployment.
FTP or git serves as a basis for the deployment. Files stored in the filesystem by the app are reset after each restart and deployment and are not available across processes since each app is isolated from other apps and processes.
For storing persistent data we recommend using MongoDB.
Each plan includes a free MongoDB storage which can be used to save sessions, uploaded images/files, etc.
See GridFS for more information.