Discussion

RFC 2045, defines a number of header fields, including Content-Type. The Content-Type field is used to specify the nature of the data in the body of the MIME entity, by giving media type and subtype identifiers, and by providing auxiliary information that may be required for certain media types. After the type and the subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute/value notation. The ordering of parameters is not significant. In general, the top-level media type is used to declare the general type of data, while the subtype specifies a specific format for that type of data. Thus, a media type of "image/xyz" is enough to tell a user agent that the data is an image, even if the user agent has no knowledge of the specific image format "xyz". Such information can be used, for example, to decide whether or not to show a user the raw data from an unrecognized subtype - such an action might be reasonable for unrecognized subtypes of "text", but not for unrecognized subtypes of "image" or "audio". For this reason, registered subtypes of "text", "image", "audio", and "video" should contain embedded information that is really of a different type. Such compound formats are represented using the "multipart" or "application" types as the RFC 2045 states.