Lines 599&600 are not deleted (as suggested in #2485) as I guess there are use cases where some gen_files require a suffix to be appended? Instead, now all source_suffix values are checked (not just the first one) before appending the suffix. This solution seems robust to me.

LGTM.

Note: I will investigate later what is happened when the first element of source_suffix is not .rst (ex. .md).

Note: I will investigate later what is happened when the first element of source_suffix is not .rst (ex. .md).

It does not work. autosummary generates files formatted by reStructredText. So it should be an extension of other formats.

I think suffix=ext[0] is not good. It should be '.rst' always.
And, it would be better to ensure .rst is included in source_suffix before calling generate_autosummary_docs() like following::

if '.rst' not in app.config.source_suffix:
    app.warn('.rst should be added to source_suffix!')
    return

Merged. I added 6b7df0c to check parsers.
Thank you for contribution