TYPO3 Core

Hidden files and folders (file- folder names beginning with a dot) are normally not shown in the filelist.
There is a user setting "Show hidden files and folders in the filelist" and if you check it, hidden files and folders are shown (as long as they are found directly in fileadmin) on the first click on the module "Filelist".
Attention: In versions older than 10 you have to logout and login first.
After that you select/click "fileadmin (auto-created)" in the page tree, but than the hidden files are gone again (you cant see them anymore).

Step by step:
1) Go to filelist -> fileadmin and create a folder called .hidden-folder
2) After you pressed enter you will not see this folder although it exist
3) Go to filelist -> fileadmin and create a file called .hidden-file
4) After you pressed enter you will not see this file although it exist
5) Go to our user settings an check "Show hidden files and folders in the filelist" + save
6) Only TYPO3 Version 8 and 9: Log out and in again
7) Click the module Filelist
8) You will see the hidden folder in the page tree and both (file and folder) on the right side
9) Click on fileadmin in the page tree
10) The hidden file and folder are not visible anymore on the right side. There is no way to rename or delete the hidden file.

Checked in TYPO3 Version 8, 9 and 10