Fix RDFa markup for file/image formatters

I just realized that the HTML output generated by certain field formatters interferes with the RDFa parsing, producing some inconsistent RDF. To explain, let's look at 2 examples.

Example 1: image field
Create an article with an image attached to it.

Full node:

<div class="field field-name-field-image field-type-image field-label-hidden clearfix">
  <div class="field-items">
    <div rel="rdfs:seeAlso" class="field-item even">
      <img src="http://localhost/image/generate/large/public/field/image/2009iswc.png"/>
    </div>
  </div>
</div>

Equivalent RDF: <> rdfs:seeAlso <http://localhost/image/generate/large/public/field/image/2009iswc.png>

Teaser:

<div class="field field-name-field-image field-type-image field-label-hidden clearfix">
  <div class="field-items">
    <div rel="rdfs:seeAlso" class="field-item even">
      <a href="/node/1"><img src="http://localhost/image/generate/medium/public/field/image/2009iswc.png"/></a>
    </div>
  </div>
</div>

Equivalent RDF: <> rdfs:seeAlso <http://localhost/node/1>

The RDF produced by the teaser is not the same as the RDF produced by the full node of the same article. The full node RDFa is good, while the teaser links to itself which is not what we expect. This glitch is due to the @href of the a tag wrapping the img tag (RDFa parsers take the first @href/@src available in the tag containing the @rel, or in the first children tag containing @href/@src).

Example 2: file field
Add a file field to a content type and create node with a txt file attached to it.

<div class="field field-name-field-file field-type-file field-label-above clearfix">
  <div class="field-label">file: </div>
  <div class="field-items">
    <div rel="rdfs:seeAlso" class="field-item even">
      <span class="file">
        <img src="/modules/file/icons/text-plain.png" title="text/plain"/>
        <a href="http://localhost/sites/default/files/swcambridge-20091208.txt">swcambridge-20091208.txt</a>
      </span>
    </div>
  </div>
</div>

Equivalent RDF:

<> rdfs:seeAlso <http://localhost/modules/file/icons/text-plain.png>
<> rdfs:seeAlso <http://localhost/sites/default/files/swcambridge-20091208.txt>

Here, the problem is that the icon for the attachment text-plain.txt is picked up by the RDFa chaining: because the last div tag does not contain any @href, @src or @resource, the objects of the mapping rdfs:seeAlso are all the children tags containing either @href/@src.

Conclusion
This only affects the fields containing a link to another resource (aka of type owl:ObjectProperty) like image and file, but does not affect text or numeric type fields.

One way to solve this is by adding a @resource attribute with the url of the file/image in the same div as the rel attribute:

<div class="field field-name-field-file field-type-file field-label-above clearfix">
  <div class="field-label">file: </div>
  <div class="field-items">
    <div rel="rdfs:seeAlso" resource="http://localhost/sites/default/files/swcambridge-20091208.txt" class="field-item even">
      <span class="file">
        <img src="/modules/file/icons/text-plain.png" title="text/plain"/>
        <a href="http://localhost/sites/default/files/swcambridge-20091208.txt">swcambridge-20091208.txt</a>
      </span>
    </div>
  </div>
</div>