fix: multiple max-height causing multiple instances of scrollbars on attachment modal

[AnkushBanerjee1999]

Brief Title

Multiple max-height causing multiple instances of scrollbars on attachment modal.

Video/Screenshots

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PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-994 after approval. Contributors are requested to replace <pr_number> with the actual PR number.

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