"Canonicalize URL.
Replace the ?/ character with a ?! character and append .png.
Also replaces special characters with `mh-url-hexify-string'
-since not all characters, such as :, are legal within Windows
-filenames. In addition, replaces * with %2a. See URL
+since not all characters, such as :, are valid within Windows
+filenames. In addition, replaces * with %2a. See URL
`http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/shell/reference/ifaces/iitemnamelimits/GetValidCharacters.asp'."
(format "%s/%s.png" mh-x-image-cache-directory
(mh-replace-regexp-in-string