The 'ImagePage' calls the 'setFileVersion' of the current 'OutputPage' object only at the end of method 'openShowImage' [1]. But this method just gets called if the page is not in 'diff' view [2].
Why is this a problem?
In core there is afaik no use of the 'fileVersion' field of 'OutputPage'. But there are extensions that rely on this information. A famous example is the FlaggedRevs extension, which uses this field to populate the review HTML form with hidden fields [3]. The described behavior results in a bad parametrized HTML form and therefore in a broken functionality.
What is a possible soultion?
One could just move the call of 'OutputPage::setFileVersion' out of 'ImagePage::openShowImage' and into 'ImagePage::view'
[1] http://git.wikimedia.org/blob/mediawiki%2Fcore.git/251b345151a1ce517d9f06f9bfa18d520a6c7e5a/includes%2Fpage%2FImagePage.php#L621
[2] http://git.wikimedia.org/blob/mediawiki%2Fcore.git/251b345151a1ce517d9f06f9bfa18d520a6c7e5a/includes%2Fpage%2FImagePage.php#L153
[3] http://git.wikimedia.org/blob/mediawiki%2Fextensions%2FFlaggedRevs.git/0057d275a4903ebd8750bbeabee4b69affecf48b/frontend%2FFlaggablePageView.php#L1138