找到答案了，不包含前面2个连字符，最大70个字符
https://stackoverflow.com/questions/8250154/html-multipart-form-maximum-length-of-boundary-string
摘录如下：
RFC2616 (HTTP/1.1) section "3.7 Media Types" says that the allowed types in the Content-Type header is defined by RFC1590 (Media Type Registration Procedure).
RFC1590 updates RFC-1521(MIME).
RFC1521 says that a boundary "must be no longer than 70 characters, not counting the two leading hyphens".
The same text also appears in RFC2046 which supposedly obsoletes RFC1521.
