Fix #55 -- await file creation coroutine

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 59.94%. Comparing base (c2b2957) to head (872aad5). Report is 1 commits behind head on main.

Files	Patch %	Lines
sam/slack.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #56 +/- ## ========================================== - Coverage 60.10% 59.94% -0.16% ========================================== Files 11 11 Lines 391 392 +1 ========================================== Hits 235 235 - Misses 156 157 +1 ```

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[sentry-io[bot]]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ BadRequestError: Error code: 400 - {'error': {'message': 'Invalid file format. Supported formats: "c", "cpp", "css... sam.slack in handle_message View Issue

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