PageViewer Web Part won't display an.shtml page

Hi-

I have a SharePoint 2010 Publishing page and on it I've placed a PageViewer web part.  It was working fine.  Then the external site we were displaying through the PageViewer was updated, so we updated the URL.  The URL is now an ".shtml" page.  The pageViewer won't display it.  I tried a different .shtml page and it also won't display that.  I tried putting a content editor web part on the page and using an IFrame tag to display the .shtml page.  Does not work either.  Could some setting in SharePoint could interfere with the pageViewer showing this page.  The page loads fine in my browser.

Thanks!