It is the HTTP server's job to tell its client the type of content being sent. This is done by the Content-Type header. The value of the Content-Type header should be a valid media type (formerly known as the MIME type) registered with the Internet Assigned Numbers Authority (IANA). See the Further reading section of this chapter for a link to the IANA list of media types.

There are a few ways to determine the media type of a file. If you're on a Unix-based system, such as Linux or macOS, then your operating system already provides a utility for this.

Try the following command on Linux or macOS (replace example.txt with a real filename):

file --mime-type example.txt

The following screenshot shows its usage:

As you can see in the preceding screenshot, the file utility told us the media type of index.html is text/html. It also said the media...