This is a sample of how to upload. You must ensure that the FILE name is exactly the same as the JPG thumbnail of the image you upload. In this example, I have sample.3ds and sample.jpg. Save as to save the file.
I had a go at uploading an object - but when it didn't at once appear I (as the images usually do) I did it again to see if there was an error and that was when I noticed the comment about validation... oops... guess I should have read that the first time around. Sorry... so please remove the duplicate.