`:UltiSnipsEdit!` doesn't list SnipMate snippet files with `g:UltiSnipsEnableSnipMate` set

Expected behavior: The command :UltiSnipsEdit! should list the private and public snippet files plus the SnipMate snippet files associated with the filetype when the variable g:UltiSnipsEnableSnipMate is set.

Actual behavior: The SnipMate snippet files are not listed even if their snippets are loaded.

Steps to reproduce

1. Under default settings, create a ~/.vim/UltiSnips/test.snippets and a ~/.vim/snippets/test.snippets with at least a snippet in each one;
2. Open a new buffer and :set ft=test;
3. g:UltiSnipsEnableSnipMate is set by default, so try to trigger a snippet from each of the files created to verify they are loaded and working;
4. Run the command :UltiSnipsEdit! (with the bang).

You should see something like the following listing in the bottom:

* 1: /home/user/.vim/UltiSnips/all.snippets
* 2: /home/user/.vim/UltiSnips/test.snippets
* 3: /home/user/.vim/plugged/vim-snippets/UltiSnips/all.snippets
Type number and <Enter> or click with mouse (empty cancels):

The SnipMate snippet file /home/user/.vim/snippets/test.snippets should be listed at the end (I think), but is not shown.

---

• Operating System: Ubuntu 20.04
• Vim Version: VIM - Vi IMproved 8.1 (2018 May 18, compiled Apr 15 2020 06:40:31) Included patches: 1-2269
• UltiSnips Version: b3fd9022bdc3472dac114a8b2aa85b845189514b (master)
• Python inside Vim: 3.8.5 (default, May 27 2021, 13:30:53) [GCC 9.3.0]
• Docker repo/vimrc: does not apply