MDL-9443 core_files: Also filter .xhtml resources.

Change the filters to treat .xhtml files the same as .html,
so they are filtered by both "All files" and "HTML files only".

lib/filelib.php

@@ -2320,7 +2320,7 @@ function send_file($path, $filename, $lifetime = null , $filter=0, $pathisstring
 
     } else {
         // Try to put the file through filters
-        if ($mimetype == 'text/html') {
+        if ($mimetype == 'text/html' || $mimetype == 'application/xhtml+xml') {
             $options = new stdClass();
             $options->noclean = true;
             $options->nocache = true; // temporary workaround for MDL-5136
@@ -2494,7 +2494,7 @@ function send_stored_file($stored_file, $lifetime=null, $filter=0, $forcedownloa
         readfile_accel($stored_file, $mimetype, !$dontdie);
 
     } else {     // Try to put the file through filters
-        if ($mimetype == 'text/html') {
+        if ($mimetype == 'text/html' || $mimetype == 'application/xhtml+xml') {
             $options = new stdClass();
             $options->noclean = true;
             $options->nocache = true; // temporary workaround for MDL-5136

mod/resource/lib.php

@@ -383,7 +383,7 @@ function resource_pluginfile($course, $cm, $context, $filearea, $args, $forcedow
 
     // should we apply filters?
     $mimetype = $file->get_mimetype();
-    if ($mimetype === 'text/html' or $mimetype === 'text/plain') {
+    if ($mimetype === 'text/html' or $mimetype === 'text/plain' or $mimetype === 'application/xhtml+xml') {
         $filter = $DB->get_field('resource', 'filterfiles', array('id'=>$cm->instance));
         $CFG->embeddedsoforcelinktarget = true;
     } else {