after enabled multisites, image upload doesn't work

i have enabled multisites on wordpress when i was using 3.0.1, and everything worked fine. after i upgraded to 3.0.4, the image upload doesn’t work. the image is saved to /blogs.dir/blogid/files/year/month/ folder, but the media library doesn’t display any images even there are a lot in that file structure. when i tried to upload a new image, it didn’t show up on the form after shows 100% saved. i tried to access the image on browser, it showed up as: https://blog.com/files/2011/02/Tulips.jpg

please help. what did i miss? thanks a lot.