Screenshots are often nice and sometimes helpful. It's almost always more useful to have a (text not image) copy of the error message you saw. I've transscribed it from the image below but please copy/paste for us next time! (if you like you can still screenshot in addition) Also, would help to have some more details/background. e.g. did you try multiple times and did this happen every time? did it happen at the beginning of the upload or end or halfway through? or anything else you can think of. Thanks!

Error: uncaught exception: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMFileRreader.result]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: http://bits.wikimedia.org/commons.wikimedia.org/load.php?debug=false&lang=en&modules=ext.uploadWizard&skin=monobook&version=20120513T022626Z&* :: :: line 93" data: no]

Yes, I tried multiple times (With Firefox 11 on Linux and 32-bit Windows 7, and Google Chrome on 32-bit Windows 7) and received the same error. Everything works fine up until the point where I select the file to use, using Firefox's built in Filechooser (at least, that's what they call it in Java), and click 'open'. A few seconds after I do that, the error messages appear, manifesting themselves in different forms based on OS and web browser:

*In Linux Firefox, the error message appears above the upload form. The "Select A Media File to Donate" button becomes unresponsive to mouse input.
*In Windows Firefox, the same actions described above will result in a wall of text being dumped to Firefox's Error Console.
*In Windows Chrome, the window outright crashes (e.g. the whole window goes gray and it reads "Oops, something went wrong..."

Hope that helps

Same goes for Firefox on iMac.

*The last working step before the upload fails: http://upload.wikimedia.org/wikipedia/commons/2/2f/UploadWizardChunkedUploadLastStepBeforeFail1500ppx.png
*An unresponsive Firefox: http://upload.wikimedia.org/wikipedia/commons/3/38/UnresponsiveUploadWizard1100ppx.png
*Nasty Error Message in error console: http://commons.wikimedia.org/wiki/File:FirefoxErrorConsoleUploadWizardChunkedUploadFail1600ppx.tiff

I suppose the problem is universal; webkit based browsers don't work with it either:

http://commons.wikimedia.org/wiki/File:UploadWizardApparentlyWorkingSafariChunkedUploads1000ppx.png

http://commons.wikimedia.org/wiki/File:UploadWizardNotUploading1600ppx.png

This is a known issue. It will be fixed as soon as https://gerrit.wikimedia.org/r/#/c/6722/ is merged and deployed.

Deployed, should be fixed.