So ... this is like the definition of pedantic, but:

The list of characters that can't be in tokens in RFC 2045 [MIME] is (https://tools.ietf.org/html/rfc2045 section 5.1):

()<>@,;:\"/[]?=

as implemented in https://github.com/golang/go/blob/master/src/mime/grammar.go .

The list of characters that can't be in tokens in RFC 2616 [HTTP/1.1] is (https://tools.ietf.org/html/rfc2616 section 2.2):

()<>@,;:\"/[]?={}

Note the extra 2 characters. This impacts mime.FormatMediaType which is advertised as usable for either RFC in that it might try to stick a curly brace into a string which is otherwise considered a token.

My particular use-case for this is

mime.FormatMediaType("attachment", map[string]string{"filename": filename}))

which is used as the value of the Content-Disposition header. So for example

attachment; filename=foo

is valid, while

attachment; filename=foo{

is not -- that one would require quoting, at least per the spec. See sample code in https://play.golang.org/p/TbkEuMijPPb .

I have no idea if any web browsers care about this. I happened upon this when trying to figure out how filename characters need to be escaped properly before finding this function in the go standard library and then comparing it to the spec.