PDF upload in Website fails

When trying to upload a PDF document on a website page (using the Attach File widget) I get the following stacktrace and the website page displays the stacktrace whenever you attempt to view it.

I have tried a number of PDFs with the same result. Other file types, e.g. Word documents upload without error. Also, PDFs upload fine within the main JSF UI.

org.nuxeo.ecm.webengine.WebException: Failed to render template: /opt/nuxeo/nuxeo-5.3.1/server/test/data/NXRuntime/web/root.war/modules/org.nuxeo.ecm.platform.webengine.sites/skin/template_default.ftl
...
Caused by: org.nuxeo.ecm.platform.rendering.api.RenderingException: freemarker.core.InvalidReferenceException: Expression mimetypeService.getMimetypeEntryByMimeType(file.file.mimeType) is undefined on line 143, column 42 in nxtheme://theme/default/*/freemarker/custom-sites/page/view.
at org.nuxeo.ecm.platform.rendering.fm.FreemarkerEngine.render(FreemarkerEngine.java:161)
at org.nuxeo.ecm.webengine.model.impl.AbstractWebContext.render(AbstractWebContext.java:582)
... 51 more