On CommonsHelper.php, if the user chooses not to upload the file directly, s/he will get text that says "Save [filename] locally, then <upload at commons>" (where the text inside <> is a button). However, the GET/POST parameter uselang=fromwikimedia is not passed like it should be, with the result that the form will tell the user, in red lettering, "If you do not provide suitable license and source information, your upload will be deleted without further notice. Thank you for your understanding." (see example page: http://commons.wikimedia.org/w/index.php?title=Special:Upload&uploadformstyle=basic&wpUploadDescription=Text%20here). This is plainly wrong: CommonsHelper has already provided the license tag. This is not only inefficient, but it's causing users who are unfamiliar with the interface to add their own license, which is harmful because sometimes they're adding the wrong license.
If the uselang=fromwikimedia parameter is sent along, they will not have the option add a license, which is how it should be (see example page: http://commons.wikimedia.org/w/index.php?title=Special:Upload&uploadformstyle=basic&wpUploadDescription=Text%20here&uselang=fromwikimedia)
I'm marking this critical because I have watched literally thousands of images be uploaded with a confusing duplicate license (best case scenario) or an entirely incorrect one (worst case scenario).
Not to be pedantic (I assume you're well familiar with HTML), but I'm going to type the exact text that should be added, so that the fix shouldn't take you more than 3 minutes. Right before the form is closed (</form>), add this text:
<input type='hidden' name='uselang' value='fromwikimedia' />