Back port to 7.8: Content blocks: Unable to save document with content blocks field in drop down mode

Back port of HIPPLUG-1001

Reproduction:

• Check out https://forge.onehippo.org/svn/content-blocks/demo/tags/content-blocks-demo-1.10.00
• Add an Upload field to existing doc type 'Overview
• Set /hippo:namespaces/defaultcontent/overview/editor:templates/_default_/cpitems/contentPickerType=dropdown
• Set /hippo:namespaces/hippo/resource/editor:templates/_default_/upload/fileupload.flashEnabled=false (boolean property)
• Create a new document and upload a field