This is a version incompatibility between f937b1f20d4e6941ce522c95d27e3dc6c33d098a (committed in January) and 75d025c973bc93d76f87abee68d003234b193d4c (committed last October). Are you using MediaViewer master with MediaWiki REL1_26?

@Tgr We are using 15793c3081ddf3901a76142dcb5a0edb9a9aa387, which was a commit made on January 19th. To the 2nd part, yes, we're using MultimediaViewer on mw.v 1.26.2.

You will have to update MediaWiki to a more recent version then, or downgrade MultimediaViewer.

@Legoktm is there a way to declare an extension only compatible with master? Would putting something like ~1.27 in extension.json work with pre-release versions?

@Tgr, what should the priority be?

I don't think there is anything to fix here. Using mismatched core and extension versions is unsupported and should be done at your own risk.

@LegoFan4000: Why did you raise the https://www.mediawiki.org/wiki/Phabricator/Project_management#Setting_task_priorities priority of this task? Has this suddenly become more urgent? Why did you assign this task georgebarnick? Did George agree with that beforehand?