HTTP does not use the Content-Transfer-Encoding (CTE) field of MIME. Proxies and gateways from MIME-compliant protocols to HTTP MUST remove any non-identity CTE ("quoted-printable" or "base64") encoding prior to delivering the response message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where "safe transport" is defined by the limitations of the protocol being used. Such a proxy or gateway SHOULD label the data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol.