Why does the content-type header in the request contain additional information beyond just the MIME type?

There have been instances where the content-type header of a request (such as one made by Firefox) includes not just information about the MIME type, but also details on encoding.

For example, when sending JSON via AJAX, instead of

application/json

(which was anticipated), Firefox sent:

application/json; charset=UTF-8

This behavior raises several questions:

  1. What are other possible "parameters" (or "options", or whatever term is used for the charset value) that could be included?
  2. Will application/json consistently be listed first, or might the values be in a different order (e.g., could it be charset=UTF-8; application/json)?
  3. Is the separator always a ;?
  4. What is the standard method to test for a specific MIME type? Simply comparing headers is insufficient. Any suggestions or improvements beyond 
    contentType === 'application/json || contentType.startsWith('application/json;')
    ?

PS: Regarding question #4, I've posted a separate inquiry. Please see Get an entire string or a substring, depending on a specific character

javascriptajaxjson

Answer №1

For a more in-depth understanding, please refer to the HTTP 1.1 RFC

Section 3.7 on Media Types explains how HTTP utilizes Internet Media Types [17] within the Content-Type (section 14.17) and Accept (section 14.1) header fields for flexible data typing and type negotiation.

   media-type     = type "/" subtype *( ";" parameter )
   type           = token
   subtype        = token Additional parameters can be included after the type/subtype as attribute/value pairs (refer to section 3.6).

To summarize:

  1. There is no predefined list of possible values
  2. The correct order is type first
  3. The separator used is ;

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