autosummary may output .md files with MyST-Parser

[kou]

Describe the bug

If we have the following configurations, autosummary generates .md files with reST content:

extensions = ['myst_parser', 'sphinx.ext.autosummary']
source_suffix = {
    '.md': 'markdown',
    '.rst': 'restructuredtext',
}
autosummary_generate = True

autosummary should generate .rst files because they use reST.

Note that if we use the following order for source_suffix, autosummary generates .md files:

source_suffix = {
    '.rst': 'restructuredtext',
    '.md': 'markdown',
}

How to Reproduce

Create a new project:

$ sphinx-quickstart /tmp/sphinx-test

Add the following configurations to conf.py:

extensions = ['myst_parser', 'sphinx.ext.autosummary']
source_suffix = {
    '.md': 'markdown',
    '.rst': 'restructuredtext',
}
autosummary_generate = True

Add the following content to index.rst:

.. currentmodule:: sphinx

.. autosummary::
   :toctree: generated/

   environment.BuildEnvironment

Generate document:

$ make -C /tmp/sphinx-test html

Check generated files:

$ ls -lh /tmp/sphinx-test/generated
total 4.0K
-rw-r--r-- 1 kou kou 1.4K Mar 21 00:13 sphinx.environment.BuildEnvironment.md

It has .md suffix not .rst suffix.

Environment Information

Platform:              linux; (Linux-6.6.15-amd64-x86_64-with-glibc2.37)
Python version:        3.11.8 (main, Feb  7 2024, 21:52:08) [GCC 13.2.0])
Python implementation: CPython
Sphinx version:        7.3.0+/3bedde26a
Docutils version:      0.20.1
Jinja2 version:        3.1.2
Pygments version:      2.17.2

Sphinx extensions

['myst_parser', 'sphinx.ext.autosummary']

Additional context

No response