override.txt doesn't work in "server" texture pack

Minetest version

5.3.0 and 5.4.0-dev-af22dd86e

OS / Hardware

Operating system: Windows 10 64-bit CPU: Intel Core 2 Duo

Summary

The override.txt file doesn't work when it's in the "server" texture pack, even when the same file works when used in a client texture pack.

This error is output to the log when trying to use it:

2021-01-03 17:12:29: INFO[Main]: Server: ignoring unsupported file extension: "override.txt"

Steps to reproduce

1. Create a "server" folder in the textures folder.
2. Create a override.txt inside that folder with the following contents:

default:dirt_with_grass top default_stone.png

3. Load into a world, and the textures for default:dirt_with_grass will not have changed.
4. Quit, and then enable the "server" texture pack in the content tab.
5. Load into a world again, and default:dirt_with_grass will now have the stone texture on top.