Additional Thumbnails/thumbnail.png returned by tika when parsing odt

Since the upgrade to Tika 2.3.0 (XWIKI-19402), and more precisely https://issues.apache.org/jira/browse/TIKA-3644 and additional "Thumbnails/thumbnail.png" is returned when calling Tika#parseToString.

We need to decide if:

• we accept Thumbnails/thumbnail.png => then close the issue as invalid
• we find a way to remove it without downgrading Tika => close the issue as fixed
• downgrade Tika, but this can only be temporary because on the long run we need to have Tika up to date

Corresponding failing test: org.xwiki.search.solr.internal.metadata.DocumentSolrMetadataExtractorTest#testAttachmentExtractFromOpenDocument