fix for 39951 r=slamm a=waterson

xpfe/components/filepicker/res/locale/en-US/filepicker.properties

@@ -2,7 +2,7 @@
 allTitle=All Files
 allFilter=*.*
 htmlTitle=HTML Files
-htmlFilter=*.html; *.htm
+htmlFilter=*.html; *.htm; *.shtml; *.xhtml
 textTitle=Text Files
 textFilter=*.txt
 imageTitle=Image Files