According to http://maven.apache.org/plugins/maven-shade-plugin/shade-mojo.html, your exclusion syntax is wrong:
Artifacts to include/exclude from the final artifact. Artifacts are denoted by composite identifiers of the general form groupId:artifactId:type:classifier. ... For convenience, the syntax groupId is equivalent to groupId:*:*:*, groupId:artifactId is equivalent to groupId:artifactId:*:* and groupId:artifactId:classifier is equivalent to groupId:artifactId:*:classifier.
So either use com.sun.jersey.contribs:jersey-multipart:*:jar
or com.sun.jersey.contribs:jersey-multipart
for your exclusion.
<artifactSet>
<excludes>
<exclude>com.sun.jersey.contribs:jersey-multipart</exclude>
</excludes>
</artifactSet>