Fix #1356 files_upload_v2 filename required even though content provided

[seratch]

Summary

This pull request resolves #1356

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

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

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 python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

Merging #1361 (5517b27) into main (f37c691) will increase coverage by 0.20%.
The diff coverage is 100.00%.

❗ Current head 5517b27 differs from pull request most recent head 8035e45. Consider uploading reports for the commit 8035e45 to get more accurate results

@@            Coverage Diff             @@
##             main    #1361      +/-   ##
==========================================
+ Coverage   85.37%   85.58%   +0.20%     
==========================================
  Files         111      111              
  Lines       11557    11557              
==========================================
+ Hits         9867     9891      +24     
+ Misses       1690     1666      -24     

Impacted Files	Coverage Δ
slack_sdk/web/internal_utils.py	78.04% <100.00%> (+11.70%)	⬆️

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