You can't access your files directly from FilesApp because of some security reasons and our scrambled file hash algorithm.
If you want access your files from FilesApp steps are simple, tap options menu which you can find where the files listed.
Tap three dots button then tap "Add to Files" after that you will see all the files in the FilesApp which you selected.
For removing from FilesApp, go to Settings and tap "Remove from FilesApp".
PS: Sharing files with FilesApp multiples your files. It means you are increasing the storage usage.