On iOS, applications cannot programmatically access files external to the application sandbox (for example, an iOS application cannot programmatically navigate to the user's picture directory and pick a file). The bridge between the local filesystem and the iOS app's sandbox was limited to the image picker controller until iOS 8. iOS 8 introduces the new document picker controller and document provider API. In this interaction model, the application implementing the document provider extension creates the document picker UI and the host application uses this provided UI to let the user select the documents to be used in the host application execution (similar to the file open picker and provider capability on the Windows Runtime platform).

UIImagePickerController

For Android, on top of the local file storage that is only app-specific, applications have access to two other locations: public and private external storage (depending on the hardware). External storage in this context...