There is a setting in XWikiPreferences but it’s set to 107374182400 bytes by default in 14.10.10 (so I would guess comparable in 14.10.x), which is about 100MB so I doubt it’s that.
See https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Attachments for this limit.
It may be the proxy config, if you don’t find anything do look for the logs of your web container (tomcat, jetty or whichever you have), there may be interesting information in there.
Anca