I might be wrong on this, but I'm thinking that the upload feature is for only for posting graphic images.
I'm also not sure of all the formats supported, but I know the following.
Bitmaps (files with the .BMP extension) are not supported because those files are way too big.
JPGs and GIFs work.
Here's an animated GIF file. Double-click on the image, to expand. What direction is she spinning?
[attachment=1:jt4o43m0]Optical Illusion (Spinning Woman).gif[/attachment:jt4o43m0]
And, here's a JPG file (just for "good measure").
[attachment=0:jt4o43m0]Optical Illusion Art (1).jpg[/attachment:jt4o43m0]