Hmmm... I'll test when I get a chance (tonight or tomorrow). This is strange. Usually during that time, it is converting the image. But if it spins forever, I think the image conversion failed, and so the spinning 100% never gets updated that it is complete. When I was first developing it, I'd seen this behavior, but it was an actual coding bug at the time. The good thing is, if this is a failure like that, logs on the server should indicate the specific issue (makes it much easier to identify and fix).
Sorry for the inconvenience. Most likely, there's something about the file format, or perhaps something weird going on with the ability to "save" the file to the hard drive on the server.