app.message listener does not handle events when a file is attached #644

When a message has attached files, the corresponding message event has file_share subtype. In this scenario, bolt-python's app.message listener does not handle the event even when the text matches.

Reproducible in:

any versions of bolt-python, any runtime

Steps to reproduce:

(Share the commands to run, source code, and project settings (e.g., setup.py))

1. Have app.message listener with keyword in your bolt-python app
2. Post a message that matches the keyword with any files

Expected result:

The event should be handled by the app.message listener

Actual result:

The event listener does not handle the event while it does if a message does not have files