The specific file permissions on files uploaded with SFTP can be set using -u(umask) for directory permission and -m for required file permission.
If the permission for an uploaded file needs to be 664 and the permission for a directory needs to be 775, then the following example can be considered.
Match User ftp_user
ForceCommand internal-sftp -u 002 -m 664
Manpage shows the following info:
Sets explicit file permissions to be applied to newly-created files instead of the default
or client requested mode. Numeric values include: 777, 755, 750, 666, 644, 640, etc.
Using both -m and -u switches makes the umask (-u) effective only for newly created
directories and explicit mode (-m) for newly created files.