Detect some HTML files as separate invalid filetype

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1808507 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/poifs/filesystem/FileMagic.java

@@ -74,6 +74,8 @@ public enum FileMagic {
     RTF("{\\rtf"),
     /** PDF document */
     PDF("%PDF"),
+    /** Some different HTML documents */
+    HTML("<!DOCTYP".getBytes(), "<html".getBytes()),
     // keep UNKNOWN always as last enum!
     /** UNKNOWN magic */
     UNKNOWN(new byte[0]);