Match if "no other but base64 encoding"

Moin,

when checking attachments, I want to sort out those eMails, which
have no other but base64 encoding. My 1st attempt is this:

        :0 BH:
        * ^content-type:.*text/plain
        * ^content-transfer-encoding:
        * ! ^content-transfer-encoding: *[^ b][^a][^s][^e]
        $BASE64

However, this matches on "content-transfer-encoding: binary", too.
But I don't see why. Either explanation why it matches or better/
optimal solution to the original problem will be very appreciated.

-- 
© Rado S. -- You must provide YOUR effort for your goal!
Even if it seems insignificant, in fact EVERY effort counts
for a shared task, at least to show your deserving attitude.

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