Fix #901 by correcting type hints for files_upload method args

[seratch]

Summary

This pull request fixes #901 by correcting the file arg's type hint.

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (Web API client)
• slack_sdk.webhook.WebhookClient (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.rtm.RTMClient (RTM client)
• slack_sdk.signature (Request Signature Verifier)
• /docs-src (Documents, have you run ./docs.sh?)
• /docs-src-v2 (Documents, have you run ./docs-v2.sh?)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python setup.py validate after making the changes.

[codecov]

Codecov Report

Merging #909 (a805288) into main (0a25de9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #909   +/-   ##
=======================================
  Coverage   75.62%   75.62%           
=======================================
  Files          35       35           
  Lines        3381     3381           
=======================================
  Hits         2557     2557           
  Misses        824      824           

Impacted Files	Coverage Δ
slack_sdk/web/legacy_client.py	35.69% <ø> (ø)

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