base64 encodings should be line-broken

[paulgriffiths]

Per section 6.8 of RFC2045 which defines the Content-Transfer-Encoding header:

The encoded output stream must be represented in lines of no more than 76 characters each. All line breaks or other characters not found in Table 1 must be ignored by decoding software.

Currently, no line breaks appear in base64-encoded output.

All base64-encoded output used in a message with a Content-Transfer-Encoding value of base64 should be broken into lines of 76 or less characters.

[paulgriffiths]

Note that RFC 8951 subsequently (in November 2020) updated RFC 7030 to require all endpoints to ignore the value in any Content-Transfer-Encoding header, and to specifically confirm that:

Senders are not required to insert any kind of white space.

[61131]

PR #31 has been raised to deprecate the Content-Transfer-Encoding header per RFC 8951