For a return or communication that will be manually handled, yes you can attach supporting documents, but for E-filed return, my software (Intuit's Proconnect Online) says right there "NOTE: Do not attach any document that needs IRS approval or attention". I am not sure if this applies to E-filed amended return which is likely to be handled manually by the IRS. Regardless, I feel it is better to put explanation directly where the IRS expects it.