Open TLS File
The TLS file extension and format is a custom Windows login screen used by Tune Styler, a desktop customization program included in TuneUp Utilities.
TuneUp Utilities, known today as AVG TuneUp, is a utility software package for Microsoft Windows designed to help manage, maintain, optimize, configure, and troubleshoot a computer system.
So in this case, the TLS file can include a background image, as well as custom fonts and buttons, and in this case, it overrides the default Windows startup screen when applied.