Revert "MDL-39832 Files: Revert MDL-39688 commit, f111746140fe80a821ae69e3bf518d950bf188a1"

This reverts commit 3960c00e3c.

lib/filelib.php

@@ -2021,7 +2021,11 @@ function readfile_accel($file, $mimetype, $accelerate) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', $lastmodified) .' GMT');
 
     if (is_object($file)) {
-        header('ETag: ' . $file->get_contenthash());
+        if (empty($_SERVER['HTTP_RANGE'])) {
+            // Use Etag only when not byteserving,
+            // is it tag of this range or whole file?
+            header('Etag: ' . $file->get_contenthash());
+        }
         if (isset($_SERVER['HTTP_IF_NONE_MATCH']) and $_SERVER['HTTP_IF_NONE_MATCH'] === $file->get_contenthash()) {
             header('HTTP/1.1 304 Not Modified');
             return;
@@ -2671,6 +2675,10 @@ function byteserving_send_file($handle, $mimetype, $ranges, $filesize) {
     // better turn off any kind of compression and buffering
     @ini_set('zlib.output_compression', 'Off');
 
+    // Remove Etag because is is not strictly defined for byteserving,
+    // is it tag of this range or whole file?
+    header_remove('Etag');
+
     $chunksize = 1*(1024*1024); // 1MB chunks - must be less than 2MB!
     if ($handle === false) {
         die;