Assigning Media Types to Binary Data in XML

1.1 Notational Conventions

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [IETF RFC 2119].

This specification uses properties from the XML Information Set, see [XML Information Set]. Such properties are denoted by square brackets, e.g. [namespace name].

This specification uses namespace prefixes that are listed in Table 1. Note that the choice of any namespace prefix is arbitrary and not semantically significant (see [XML Information Set]).

Namespace names of the general form "http://example.org/..." and "http://example.com/..." represent application or context-dependent URIs [IETF RFC 2396].

1.2 Requirements

This section describes the set of requirements that this document addresses.

1. Define how to indicate the media type of an XML element content whose type is xs:base64Binary or xs:hexBinary. This is meta-data that may be, but not required to, used by tools to infer the specific media type of binary data.

2. Define how to indicate the expected media type(s) of XML element content whose type is xs:base64Binary or xs:hexBinary in XML Schema. This information is needed to define the set of media types that a binary data may have. For example, a Web services application may be willing to indicate that the binary data represents an image, but leaves it to a document to further specify whether it is "jpeg", "gif", etc. This meta-data is not required to be present.

3. Define the acceptable format of media type values.

4. Define the relationship between the expected and the actual value of the media types declared for binary data in XML documents.

2.1 contentType attribute

The contentType attribute information item has the following Infoset properties:

• A [local name] of contentType.

• A [namespace name] of "http://www.w3.org/2004/11/xmlmime".

The type of the contentType attribute information item is xs:string.

2.2 expectedMediaType attribute

The expectedMediaType attribute information item has the following Infoset properties:

• A [local name] of expectedMediaType.

• A [namespace name] of http://www.w3.org/2004/11/xmlmime.

The type of the expectedMediaType attribute information item is xs:string.

The value and the meaning of the expectedMediaType attribute is similar to the value allowed for the 'Accept' header defined by HTTP 1.1 specification, Section 14.1 [IETF RFC 2616] and MUST follow the production rules defined in that section. The 'q' parameter defined by HTTP 1.1 specification, Section 3.9 [IETF RFC 2616] is allowed, but other accept-extensions are not allowed.

The expectedMediaType attribute is intended to be used as part of XML Schema annotation for a binary element information item declaration (see 3 Declaring media types for binary data).

3.1 Role of expectedMediaType Schema annotation attribute

The expectedMediaType attribute is used for annotating XML Schema to indicate the expected range of media types of the binary element content and the expected range of values for contentType attribute.

The value of the contentType attribute, if present, SHOULD be within the range specified by the expectedMediaType annotation attribute, if specified in the schema. When the expectedMediaType annotation attribute has a wildcard ("*") or a list of acceptable media types, the schema SHOULD require the contentType attribute to be present.

Applications that need to specify expected media types SHOULD use the schema annotation to declare the range of expected values. expectedMediaType annotation attribute MAY be used in conjunction with the declaration of binary element information items or with complex type definitions that are derived from xs:base64Binary or xs:hexBinary in XML Schema. If the expectedMediaType annotation attribute is used in both the binary element information item declaration as well as definition of the complex type which the binary element information item belongs to, then the expected range of values defined for the binary element information item MUST be a subset of the expected range of values defined for the complex type.

The example below consists of a type definition, PictureType, and an element declaration, Picture. The xmlmime:contentType attribute is required to be present and specifies the media type of the binary content. The schema annotation attribute xmlmime:expectedMediaType specifies that the media type of the binary content is 'image', but the subtype name is unknown.

<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:tns="http://example.com/wildcard"
           xmlns:xmlmime="http://www.w3.org/2004/11/xmlmime"
           targetNamespace="http://example.com/wildcard">

    <xs:import namespace="http://www.w3.org/2004/11/xmlmime"
            schemaLocation="http://www.w3.org/2004/11/xmlmime"/>


    <xs:complexType name="PictureType">
       <xs:simpleContent>
           <xs:restriction base="xmlmime:base64Binary" >
               <xs:attribute ref="xmlmime:contentType" use="required" />
           </xs:restriction>
       </xs:simpleContent>
    </xs:complexType>

    <!-- This element designates the range of values 
         that the element definition will accept    -->
    <xs:element name="Picture" type="tns:PictureType" 
            xmlmime:expectedMediaType="image/*"/> 
    </xs:element>

</xs:schema>

        

The example document instance below conforms to the element declaration of Picture and specifies that the binary content is of type "image/png".

<?xml version="1.0" ?>
<Picture xmlns="http://example.com/wildcard"
           xmlns:xmlmime="http://www.w3.org/2004/11/xmlmime"
           xmlmime:contentType="image/png">/aWKKapGGyQ=</Picture>
        

4.1 Binary data with known media type

The example below consists of a binary element whose media type is known in advance to be "image/jpeg".

In this example, the application fixes the media type by declaring it with an annotation in conjunction with the complex type definition.

<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:tns="http://example.com/know-type"
           xmlns:xmlmime="http://www.w3.org/2004/11/xmlmime"
           targetNamespace="http://example.com/know-type">

    <xs:import namespace="http://www.w3.org/2004/11/xmlmime"
            schemaLocation="http://www.w3.org/2004/11/xmlmime"/>


    <xs:complexType name="JPEGPictureType" 
            type="xs:base64Binary"
            xmlmime:expectedMediaType="image/jpeg"/> 

    <xs:element name="JPEGPicture" type="tns:JpegPictureType"/>

</xs:schema>

        

4.2 Binary data with preferred media type

This example illustrates that binary data with media type 'image/jpeg' is preferred but binary data with media type of 'image/tiff' is also allowed (with a lower preference).

<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:tns="http://example.com/preferred-type"
           xmlns:xmlmime="http://www.w3.org/2004/11/xmlmime"
           targetNamespace="http://example.com/preferred-type">

    <xs:import namespace="http://www.w3.org/2004/11/xmlmime"
        schemaLocation="http://www.w3.org/2004/11/xmlmime"/>


    <xs:complexType name="JPEGPreferredPictureType">
        <xs:simpleContent>
            <xs:restriction base="xmlmime:base64Binary" >
                <xs:attribute ref="xmlmime:contentType" use="required" />
            </xs:restriction>
        </xs:simpleContent>
    </xs:complexType>

    <xs:element name="JPEGPeferredPicture" type="tns:JPEGPreferredPictureType"
            xmlmime:expectedMediaType="image/jpeg;q=1.0, image/tiff;q=0.8"/> 

</xs:schema>