Dropoff at the file stage of the UploadWizard contribution funnel is steep (71.2% as of 2014-09-18; i.e. 3 in 10 users who arrive to the file upload page don't continue to the license selection page). Many theories exist for why this is the case, but as of this writing, there is little data pointing to which problems are the primary causes.
More information can be found here:
https://www.mediawiki.org/wiki/UploadWizard/Funnel_analysis