Before your files can appear in the Files app, you must indicate that your app supports Open in Place and File Sharing Enabled. These options are configured using keys in your Info.plist file. The first key is UIFileSharingEnabled, which enables iTunes sharing of files in your Documents folder. The second key is LSSupportsOpeningDocumentsInPlace, which grants the local file provider access to files in your Documents folder. Add these keys to your Info.plist and set their values to YES.

This blog contains my research com.apple.system.privilege.