(At the moment of writing I have solved the issue)
Having migrated from GlassFish to Caucho Resin, I observed that file uploading doesn't work. I have enabled logging wherever I thought it to be relevant:
Useful links were:
I have solved the problem by putting <multipart-form enable="false"/> into my resin-web.xml. The complete contents of my resin-web.xml are:
It seems to be not Resin-specific issue. Resin just implements Servlet 3.0 API.
But <multipart-form enable="false"/> seems to be specific to Resin because it resides inside Resin XML namespace.
So I think XWiki should either contain this resin-web.xml or use Sevlet 3.0 API when available.