File extension check ignores trailing characters

The util_check_valid_extension function will ignore any characters after a matched file extension, so that xsl, xslt and xslfoooo will all return XSLT_CONTENT, even though the last one really should not.

Additionally there is a check for htm and after that another one for html, but the first check will always match even in the case of html, so that code is actually useless and never execute.