IRS form 8453 Instructions, page 2, uses the term 'attach' but does not specify staple, paper-clip or loose sheets.
Use Form 8453 to send any required paper forms or supporting documentation listed next to the checkboxes on Form 8453 (don’t send Form(s) W-2, W-2G, or 1099-R).
CAUTION Don’t attach any form or document that isn’t shown on Form 8453 next to the checkboxes. If you are required to mail in any documentation not listed on Form 8453, you can’t file the tax return electronically.
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