If there is a problem with control/badmimetypes, qmail-badmimetypes complains and leaves control/badmimetypes.cdb alone.
qmail-badmimetypes ensures that control/badmimetypes.cdb is updated atomically, so qmail-smtpd never has to wait for qmail-badmimetypes to finish. However, qmail-badmimetypes makes no attempt to protect against two simultaneous updates of control/badmimetypes.cdb. For convenience, qmail-badmimetypes allows comments (lines starting with '#') and copies only the significant leading characters to control/badmimetypes.cdb.
The binary control/badmimetypes.cdb format is portable across machines.