Avoid naive detection of text/turtle and application/rdf+xml

It seems to match even on this t2flow because it includes an annotation:

<workflow xmlns="http://taverna.sf.net/2008/xml/t2flow" version="1" producedBy="taverna-2.4.0"><dataflow id="e1ddbfc6-afa0-4d0d-8616-add886157e48" role="top"><name>Workflow50</name><inputPorts><port><name>Ontology_input</name><depth>0</depth><granularDepth>0</granularDepth><annotations><annotation_chain encoding="xstream"><net.sf.taverna.t2.annotation.AnnotationChainImpl xmlns="">
  <annotationAssertions>
    <net.sf.taverna.t2.annotation.AnnotationAssertionImpl>
      <annotationBean class="net.sf.taverna.t2.annotation.annotationbeans.ExampleValue">
        <text>@prefix rdf: &lt;http://www.w3.org/1999/02/22-rdf-syntax-ns#&gt; .
@prefix rdfs: &lt;http://www.w3.org/2000/01/rdf-schema#&gt; .
@prefix skos: &lt;http://www.w3.org/2004/02/skos/core#&gt; .
Test media guess on t2flow with text/turtle annotations

.. should NOT be matched as text/turtle or application/rdf+xml