I believe this problem is sometimes seen when using Internet Explorer as your web browser. Possible solutions include:
- viewing MobileRead in compatibility mode (the icon of the broken page, next to the reload button)
- switching browsers to, well, any other browser. Firefox and Google Chrome are both great browsers.
- renaming attachment.php to anything.epub -- since it seems from the 538 KB filesize that the ebook was downloaded, but with the wrong name, renaming it should let you open it.