Safari made changes to their file upload header boundaries in their v3 version, a change was made to address this in ShopSite v9.0.2, see the ShopSite Knowledgebase article at:
http://support.shopsite.com/kbase/view_answer.php?questionID=S5526
which says:
* fixes an upload issue with the Safari v3x web browser caused by Safari's changes to the length of their MIME-encoded boundary marker. This Safari v3x browser change affected any file uploads to ShopSite including Image Upload, Database Upload, Digital Download file uploads, custom template file uploads, etc.
So you can either go back to a pre-v3 version of Safari, or upgrade your ShopSite to v9.0.2 or newer, or use a different browser such as Firefox or (gag) Internet Explorer when you are going to be uploading files through ShopSite.
-Loren