If it's the thread I started upload works ok as long as I attempt to upload the file at the same time that I create the thread. Any other time I don't get any error message such as "file too big" (it's only 124K) or the options where to place in the post. It just does nothing at all.
I had a lot of problems uploading files. It turned out the problem was a Firefox plugin, Ghostery. Try disabling all plugins, then clear your cache and try the upload again. I finally got pissed enough at Firefox that I just switched to Chrome. I haven't had a problem since.