[Minor] Improve lua_magic HTML content type check

Types which are used for html (xhtml) attachments:
text/html
application/html
application/xhtml+xml

Type which cannot be found in the wild:
text/htm

lualib/lua_magic/heuristics.lua

         end
       end
 
-      if mtype == 'text' and (msubtype == 'html' or msubtype == 'htm') then
+      if (mtype == 'text' or mtype == 'application') and (msubtype == 'html' or msubtype == 'xhtml+xml') then
         return 'html',21
       end
 
   return 'pdf',weight
 end
 
-return exports
+return exports