Fix markdown error in attaching-files.md

[horyu]

Why:

Closes: None

Fix a minor markdown rendering glitch in the "Attaching files" document. The misplaced backticks around .xls and .xlsm caused the inline code blocks to parse incorrectly on the live site.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Fixed the backtick formatting for Microsoft Office file extensions in content/get-started/writing-on-github/working-with-advanced-formatting/attaching-files.md.

• Before: ... , .xls`` ,.xlsm
• After: ... , .xls, .xlsm (properly enclosed in backticks)

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Table of review links

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Source	Review	Production	What Changed
get-started/writing-on-github/working-with-advanced-formatting/attaching-files.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Copilot]

Pull request overview

Fixes an inline-code Markdown rendering issue in the “Attaching files” documentation by correcting the backtick/comma formatting around Microsoft Office file extensions, including a version-gated .xlsm entry.

Changes:

• Corrected inline code formatting for .xls/.xlsm in the supported “Microsoft Office documents” list item.
• Ensured the version-conditional .xlsm extension renders correctly without breaking surrounding inline code.