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How did I fix «Root composer.json requires fooman/emailattachments-m2 3.6.1, it is satisfiable <…> from <…> (https://repo.packagist.org) <…> but <…> (https://repo.magento.com) has higher repository priority»?
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rm -rf composer.lock
composer clear-cache
composer remove fooman/emailattachments-m2 --ignore-platform-req=ext-ftp
composer require fooman/emailattachments-m2:3.6.1 --ignore-platform-req=ext-ftp