Fix: files_upload_v2 with channel.

[dblock]

Fixes files_upload_v2 with a single #channel parameter, see #540 (comment).

[coveralls]

Pull Request Test Coverage Report for Build 13415669916

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.381%

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Totals
Change from base Build 13314730319:	0.0%
Covered Lines:	5788
Relevant Lines:	6404

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💛 - Coveralls