Content types also known as MIME type or media types are a two part identifier for file formats. The HTTP header Content-Type is responsible for telling the HTTP client or server what type of data is being sent.


Two main headers are involved when it comes to content types

  • Accept - When a HTTP client requests data from a server it can send a comma separated list of media types. For example the headers value could be text/html, text/plain. This hints to the server what the client is looking for. Basically the client is asking the server to respond with text/html and if it cannot handle that try responding with text/plain. Whether or not the server can handle or will honor this is up to the server.
  • Content-Type - The content type header tells the client or server what format the data is being transferred in. If the client asked for text/html and the server handled it properly the data should come back with the Content-Type: text/html header. This header is how your browser knows when to render the html vs just displaying raw text. This is also used for images / files.
$ curl -v
* Rebuilt URL to:
*   Trying
* Connected to ( port 80 (#0)
> GET / HTTP/1.1
> Host:
> User-Agent: curl/7.49.1
> Accept: */*
< HTTP/1.1 200 OK
< Date: Tue, 10 Jan 2017 02:42:41 GMT
< Expires: -1
< Cache-Control: private, max-age=0
< Content-Type: text/html; charset=ISO-8859-1

Notice the Accept: */*, This means accept any format. The Content-Type: text/html header tells the client the server responded with HTML.

Media Types

  • text/plain - Simple Raw text
  • text/html - Standard HTML, when a browser gets this media type it knows to render the page as HTML instead of raw text. This includes fetching external files such as javascript, css, and images.
  • application/octet-stream - A binary file format often used to download files. If a browser gets this media type it automatically downloads the file instead of trying to display / render it.
  • application/json - The JSON data format.
  • application/pdf - PDF file.
  • image/[png,jpeg,gif] - Standard image formats.

These are only some of the formats. As you can see the difference between rendering html vs showing raw text vs downloading a file is all controlled with a simple header. Handling content types with Undertow