Hmmm. I'm not a real HTML expert, but when I look at the page's source, I see this:
<inputtype="submit" class="button" value="Post Quick Reply" accesskey="s" title="(Alt + S)" name="sbutton" tabindex="2" id="qr_submit" onclick="clickedelm = this.value" />
<inputtype="submit" class="button" value="Go Advanced" accesskey="x" title="(Alt + X)" name="preview" tabindex="3" id="qr_preview" onclick="clickedelm = this.value" />
This seems to indicate that Alt-X would execute the Go Advanced button, but when I press Alt-X, it brings up a message about sending a Quick Reply, which is the same message I get when pressing Alt-S.
When I click Go Advanced, I get this:
Since it doesn't happen for you, I guess it's either a security setting in IE8 or something in Win7. I managed to attach a file to my app message by using my tablet's email program.