SFTP in general does not have a mechanism supported in the protocol for umask. At the server level there are workarounds using a script wrapper to wrap the sftp forked process setting a umask in the wrapper script but that is not an option for the AWS Transfer Family SFTP, the architecture does not allow it.
The best option is to open a customer case to request a feature added to implement EFS umask support within a server managed by AWS Transfer Family.
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