For people who have issues uploading file from safari:
you will need to set Safari 7 to allow the Silverlight plug-in to run in unrestricted mode specifically on the website where you are getting this error.
Open Safari and go to the designer page
Click on the word "Safari" in the top left of the menu bar, next to the Apple icon
Select the Security tab
Beside Internet plug-ins, make sure there is a check mark next to "Allow Plug-ins"
Click on the "Manage Website Settings" button
Click on Silverlight in the list of available plug-ins on the left
On the right, select the site where you are having issues
To give Silverlight unrestricted access to computer resources when accessing that site, select "Run in Unsafe Mode"
Click the Trust button if you are prompted to do so.
The issue should be fixed, if everything went according to plan.