What are the valid characters for a Mime Multipart message ContentId “CID:”?
问题 From reading the RFC it appears that CID can/must only contain characters from the same set as those permissable by a regular URI. Is this correct. Im asking because I wish to writeup a simple helper that takes a CIDs prefix and adds a counter when generating CID for mime multitypes attachments. 回答1: The Content-ID value is required to be in the form of an RFC-822 addr-spec (user@domain). Since an addr-spec may contain characters not allowed in a URL, those characters are hex-encoded when