ModeShape's REST service registers with RestEASY a bunch of provider classes: https://github.com/ModeShape/modeshape/blob/master/web/modeshape-web-jcr-rest/src/main/java/org/modeshape/web/jcr/rest/JcrApplication.java.

Among those, there is a POJO which is not a real provider (and not annotated as such): org.modeshape.web.jcr.rest.form.FileUploadForm. This causes RestEASY to issue the following warning:

15:23:17,165 WARN  [org.jboss.resteasy.spi.ResteasyDeployment] (ServerService Thread Pool -- 52) Application.getClasses() returned unknown class type: org.modeshape.web.jcr.rest.form.FileUploadForm

This POJO should be removed from the list of providers.