Here on the meta site, I posted an answer to: Name for the chatroom.
When tried to upload an image, using the Internet Explorer 8, I got, both for png and jpg format, the error message:
Failed to upload; not a valid image (jpeg, png, tiff, gif or bmp)
However, uploading from the web worked. Is it a bug, or might be requirements not matched? The image was 9 kb in size and saved in a format matching the file name extension. Now I noticed the same on the main site.
Screenshot, uploaded via a web site, since this did not work either:
Finally, I tested with Mozilla Firefox 20.0.1, that was working. However, I still wanted to post this question, perhaps somebody noticed the same issue with another browser or version, and it may be relevant for beta sites in general.