phil ringnalda

For some reason, I ended up with the MSDN article on MIME type ignoring open long enough that I actually read it. If you aren’t familiar with this misfeature, well, it’s a “everyone’s at fault” thing where Apache serves up files with unknown extensions with a default MIME type, rather than with no MIME type at all, so Internet Explorer only rarely believes the server-provided MIME type, and instead looks at the content and decides what it really is.

As a result, non-IE users quite often provide examples of things like HTML or RSS or Movable Type templates by just adding .txt to the filename so they will be served as text/plain, and IE blissfully displays them as HTML or XML, or refuses to display them because they aren’t valid XML.

There are 26 MIME types that IE “knows,” plus the two it doesn’t trust: text/plain and application/octet-stream. If your type isn’t any of those, IE will accept it without munging. If your type is one of the 28 special types, IE will decide for itself whether to believe you or not. The interesting part is that it decides by looking at just a buffer containing up to the first 256 bytes of the file. If you are serving up a file that contains angle brackets as text/plain, and you can somehow avoid using anything that makes IE think it’s really something else for 256 bytes, then you can avoid having IE users email you to say that your example can’t be displayed.

The very nice thing is that comments don’t count against text. Start with a comment over 256 bytes long and you can do anything you like in the rest of the file, and IE will still display it as text. Unfortunately, you can’t put anything before the XML declaration that way, so I don’t know of a way to stop IE from trying to interpret XML-as-text as XML, but for HTML, or PHP source that otherwise might look like HTML, a nice fat comment to start will fool IE into thinking you actually know what you’re talking about.