Wikimedia Bugzilla is closed!

When $wgUseFileCache is enabled, image pages are cached, too. This fails when there are multi-page documents, because  
HTMLFileCache doesn't look at the "page" parameter and serves the same HTML-page for any page in the document.

The most simple fix to this problem is to just not cache Image pages:
--- a/includes/Article.php
+++ b/includes/Article.php
@@ -2473,6 +2473,7 @@ class Article {
                        and ($wgUser->isAnon())
                        and (!$wgUser->getNewtalk())
                        and ($this->mTitle->getNamespace() != NS_SPECIAL )
+                       and ($this->mTitle->getNamespace() != NS_IMAGE )
                        and (empty( $action ) || $action == 'view')
                        and (!isset($oldid))
                        and (!isset($diff))

I'm sure there are better solutions, but this one fixes the problem for me.

Footnote: I'm seeing this problem on a modified system with PDFs enabled, but I'm pretty sure it's the same with DjVu-Files.