x-前缀用于非标准MIME类型(即未在IANA中注册).因此，我假设image/x-png在MIME类型标准化之前就已经是PNG.

The x- prefix is given to non-standard MIME types (i. e. not registered with IANA). So I assume that image/x-png would have been PNG before the MIME type was standardized.

6.3.新的内容传输编码

实施者可以在必要时定义私有的Content-Transfer-Encoding值，但必须使用x令牌(以"X-"为前缀的名称)表示其非标准状态，例如e.例如，"Content-Transfer-Encoding: x-my-new-encoding".其他标准化的Content-Transfer-Encoding值必须由标准跟踪RFC指定. 这些规范必须满足的要求在 RFC 2048 中给出. 因此，除以下内容外，所有内容传输编码名称空间 以"X-"开头的内容已明确保留给IETF，以备将来使用 使用.

Implementors may, if necessary, define private Content-Transfer-Encoding values, but must use an x-token, which is a name prefixed by "X-", to indicate its non-standard status, e. g., "Content-Transfer-Encoding: x-my-new-encoding". Additional standardized Content-Transfer-Encoding values must be specified by a standards-track RFC. The requirements such specifications must meet are given in RFC 2048. As such, all content-transfer-encoding namespace except that beginning with "X-" is explicitly reserved to the IETF for future use.

- RFC 2045 —多用途Internet邮件扩展，第6.3节 >

这也记录在PNG规范中.参见 FalseVinylShrub的答案.

This is also documented in the PNG specification. See FalseVinylShrub's answer.