Java Network Programming, Second Edition

Guessing MIME Types

If this were the best of all possible worlds, every protocol and every server would use the MIME typing method to specify what kind of file it was transferring. Unfortunately, that’s not the case. Not only do we have to deal with older protocols, such as FTP, that predate MIME, but also many HTTP servers that should use MIME either don’t provide MIME headers at all, or they lie and provide headers that are incorrect (usually because the server has been misconfigured). The URLConnection class provides two static methods to help programs figure out the MIME type of some data; you can use these if the content type just isn’t available, or if you have reason to believe that the content type you’re given isn’t correct. The first of these is URLConnection.guessContentTypeFromName( ) :

protected static String guessContentTypeFromName(String name)

This method tries to guess the content type of an object based upon the extension in the filename portion of the object’s URL. It returns its best guess about the content type as a String. This guess is likely to be correct; people follow some fairly regular conventions when thinking up filenames. It’s unfortunate that guessContentTypeFromName( ) is protected. It’s useful for any class that needs to deal with MIME types (for example, mail clients and HTTP servers), not just for URLConnection.

The guesses are determined by the content-types.properties file, probably found in your jre/lib directory. On Unix, Java may also look ...