Actually this request:

`$.post('/api/content/reprocessPreviews', {'content_resourceSubType': 'file', 'revision_mime': 'application/pdf'});`

Gives the same error. My steps to reproduce (from a clean system, no data):

1. Reset to oaeproject/Hilary@master and oaeproject/3akai-ux@master
2. Login and upload a docx file and a pdf file
3. Verify the image-based previews are generated
4. Merge in simong/Hilary@pdf2html and simong/3akai-ux@pdf2html
5. Restart OAE
6. Run the POST above in global admin server

I get the above error.