Wiki: can't create a reStructuredText file via the UI

Summary

If a file with the extension .rst or .rest is pushed, it renders as expected.

However, it's not possible to create a file with that format via the UI.

Steps to reproduce

  1. Try to create a reStructuredText file via the UI.
  2. Note that only Markdown, RDoc, and AsciiDoc are the only options.

Example Project

https://gitlab.com/mlapierre/test/wikis/

What is the current bug behavior?

A reStructuredText file can be pushed, but it can't be created via the UI.

What is the expected correct behavior?

reStructuredText is one of the format options.

Output of checks

This bug happens on GitLab.com

/cc @DouweM @jramsay @andr3