http-head-content-type

Content-Type is one component of HTTP header fields, both request header and response header. In HTTP(Hypertext Transfer Protocol), severial header fields like Content-Type define the operating parameters of an HTTP transaction.

The header fields are tarnsmitted after the request or response line, which is the first line of a message. The end of the headers is indicated by an empty field. A core set of fields is standardlized by the IETF in RFC and must be implemented by all HTTP-compliant protocol implementations. Non-standard header fields were conventionally marked by prefixing the field name with X-. There are no limits to the size of each header field name or value or to the number of headers in standard itself. However, most servers, clients and proxy software impose some limits for practical and security reasons.Apache 2.3 by default limits each header size to 8190 bytes and there can by at most 100 headers in a single request.

Content-Type is the MIME type of the body of the request which is used with POST and PUT requests. Let's see some examples:

Request header

Content-Type: application/x-www-form-urlencoded

Content-Type: multipart/form-data; boundary=AaB03x

Also in request response header, Content-Type indicate the MIME type of response content.

Response header

Content-Type: text/html; charset=utf-8

An Internet media type is a two-part identifier for file formats on the Internet. The identifiers were originally defined in RFC 2046 for use in email send through SMTP, but their use expanded to other protocols such as HTTP. These types are called MIME type and are sometimes referred to as Content-types.The original name MIME type referred to usage to identify non-ASCII parts of email messages composed using the MIME(Multipurpose Internet Mail Extensions) specification.

A media type is composed of two or more parts, a type, a subtype and zero or more optional parameters. For example, subtypes of text have an optional charset parameter that can be included to indicate the character encoding, and subtypes of multipart type often define a boundary between parts.

Web browsers also support various media types. This enables the browser to display or output files that are not in HTML format. Media type specification is also an important information source for search engines for the classification of data files on the web.

Prior to RFC 6648, experimental or no-standard media types were prefixed with x-.Subtypes that begin with vnd. are vendor-specific, subtypes that begin with prs. are int the personal or vanity tree. New media types can be created with procedures outlined in RFC 4288.

There are a list of common media types and list of common media subtype prefixes

application/x-www-form-urlencoded

This content type is the default content type of POST method. Forms submitted with this content type must be encoded as follows:

Control names and values are escaped. Space characters are replaced by +, and then reserved characters are escaped as described in RFC1738. Non-alphanumeric characters are replaced by %HH, a percent sign and two hexadecimal digits representing the ASCII code of the character. Line breaks are represented as CR LF pairs(ie., %0D%0A).
The control names/values are listed in the order they appear in the document. The name is separated from the value by = and name/value pairs are separated from each other by &.

multipart/form-data

This type defined in RFC 2388 for additional information about file uploads. The content type application/x-www-form-urlencoded is inefficient for sending large quantities of binary data or text containing non-ASCII characters. The content type multipart/form-data should be used for submitting forms that contain files, non-ASCII data, and binary data.

The content multipart/form-data follows the rules of all multipart MIME data streams as outlined in RFC2045. The definition of multipart/form-data is avaiable at the IANA registry.

A multipart/form-data message contains a series of parts, each representing a successfull control. As with all multipart MIME types, each part has an optional Content-Type header that defaults to text/plain. User agents should supply the Content-Type header, accompanied by a charset parameter.

Each part is expected to contain:

a Content-Disposition header whose value is form-data.
a name attribute specifying the control name of the corresponding control.Control names original encoded in non-ASCII character sets may be encoded using the method outlined in RFC2045.

Examples

upload a file request header example

    Content-Type: multipart/form-data; boundary=AaB03x

    --AaB03x
    Content-Disposition: form-data; name="inputname1"

    InputText
    --AaB03x
    Content-Disposition: form-data; name="fileinputname"; filename="file.txt"
    Content-Type: text/plain

    ... contents of file1.txt ...
    --AaB03x--

upload a file and a image example

    Content-Type: multipart/form-data; boundary=AaB03x

    --AaB03x
    Content-Disposition: form-data; name="inputname1"

    InputText
    --AaB03x
    Content-Disposition: form-data; name="files"
    Content-Type: multipart/mixed; boundary=BbC04y

    --BbC04y
    Content-Disposition: file; filename="file1.txt"
    Content-Type: text/plain

    ... contents of file1.txt ...
    --BbC04y
    Content-Disposition: file; filename="file2.gif"
    Content-Type: image/gif
    Content-Transfer-Encoding: binary

    ...contents of file2.gif...
    --BbC04y--
    --AaB03x--

For detail you can see from W3C.

At last I want to tell you a situation I have ever meet with which is the purpose to write this article. I write a API for a APP developer to use, I use php to develop my api, and get post parameters with $_POST as we phper knows. But what's strange, I can not get his post parameters within $_POST array. When I use curl -d, It's ok. So I think of Content-Type, I want to compare different content type with curl -H command. When I use multipart/form-data header, ofcourse API can not get the post parameters. So I judge it must be the APP developer request my api with error Content-Type header, I tell he to add a header Content-Type: application/x-www-form-urlencoded before post my api, then it is ok. It tell us, do not use any http libs which you search from internet without rewrite it. It may be some bugs in it, especially searching with Baidu and chinese languages.

Too young too simple and too naive