When uploading a file at Commons using Special:Upload, the page cannot obtain a thumbnail of the file selected for uploading -- the throbber just spins continuously. However, if all the fields are filled out, the file uploads normally.
I encountered the issue while uploading JPEG files using the latest version of Mozilla Firefox. Not sure when the issue started – I noticed it a few days ago.
Special:UploadWizard occasionally fails to obtain thumbnails and displays "broken picture" images, but I don't know if this has the same cause as this current issue with Special:Upload.
The fix will roll out to Wikimedia wikis over the course of this week, see https://www.mediawiki.org/wiki/MediaWiki_1.26/Roadmap for details. I don't think this is critical enough to backport now, but if you do, it can be done.