Apple really doesn't want you to access the files on your iOS devices, but it has been relatively painless up until now to get around that. iOS 8.3 apparently ups the ante, breaking file access for most third-party explorer apps.
A sandboxing restriction in iOS 8.3 blocks access to directories on iOS devices that previously wasn't there. Popular file explorer app, and frequent Lifehacker pick, iFunBox noted the issue over the weekend via its Facebook page stating that Apple has blocked access to all directories. This sucks if you're either a fan of looking under the hood of iOS, or simply using it for drag and drop backup of apps, settings and data generally.
A workaround that allows access to apps that have iTunes File Sharing enabled was quickly released over the weekend, but for now it looks as though Apple and the file explorer apps community will be involved in another game of cat and mouse around file access.
As MacRumors notes, the issue isn't a factor on jailbroken iOS devices, as a jailbroken device enables OpenSSH access.