Why can't you upload files using the flash uploader?
From Joomla! Documentation
Revision as of 16:26, 1 September 2012 by JoomlaWikiBot
Sometimes the flash uploader does not work.
There are known issues with the Flash Uploader and Flash 10. At this time it is recommended to deactivate the Flash Uploader in the Main Configuration Settings and use the traditional and safe form uploader.