How to Access Document Directory Content in Files App?

From iOS 11, If you set LSSupportsOpeningDocumentsInPlace to YES and UIFileSharingEnabled to YES in your Info.plist, you can access your application Documents directory in the Files application.

<key>LSSupportsOpeningDocumentsInPlace</key>
<true/>
<key>UIFileSharingEnabled</key>
<true/>

For debugging purpose, before iOS 11 you can use FLEX. FLEX also contains a ton of useful debugging features like network request monitor, sqlite database browser etc.