diff --git a/modules/image/image.module b/modules/image/image.module
index fac8de95..73a3cae 100644
--- a/modules/image/image.module
+++ b/modules/image/image.module
@@ -833,8 +833,20 @@ function image_style_deliver($style, $scheme) {
       file_download($scheme, file_uri_target($derivative_uri));
     }
     else {
-      $headers = module_invoke_all('file_download', $image_uri);
-      if (in_array(-1, $headers) || empty($headers)) {
+      $headers = array();
+      foreach (module_implements('file_download') as $module) {
+        $function = $module . '_file_download';
+        $result = $function($image_uri);
+        if ($result == -1) {
+          // Throw away the headers received so far.
+          $headers = array();
+          break;
+        }
+        if (isset($result) && is_array($result)) {
+          $headers = array_merge($headers, $result);
+        }
+      }
+      if (empty($headers)) {
         return MENU_ACCESS_DENIED;
       }
       if (count($headers)) {
diff --git a/modules/image/image.test b/modules/image/image.test
index 3591979..6afdaf8 100644
--- a/modules/image/image.test
+++ b/modules/image/image.test
@@ -184,6 +184,22 @@ class ImageStylesPathAndUrlTestCase extends DrupalWebTestCase {
   }
 
   /**
+   * Test that we do not pass an array to drupal_add_http_header.
+   */
+  function testImageContentTypeHeaders() {
+    $files = $this->drupalGetTestFiles('image');
+    $file = array_shift($files);
+    // Copy the test file to private folder.
+    $private_file = file_copy($file, 'private://', FILE_EXISTS_RENAME);
+    // Tell image_module_test module to return the headers we want to test.
+    variable_set('image_module_test_invalid_headers', $private_file->uri);
+    // Invoke image_style_deliver so it will try to set headers.
+    $generated_url = image_style_url($this->style_name, $private_file->uri);
+    $this->drupalGet($generated_url);
+    variable_del('image_module_test_invalid_headers');
+  }
+
+  /**
    * Test image_style_url().
    */
   function _testImageStyleUrlAndPath($scheme, $clean_url = TRUE, $extra_slash = FALSE) {
diff --git a/modules/image/tests/image_module_test.module b/modules/image/tests/image_module_test.module
index 8a322fb..fc66d9b 100644
--- a/modules/image/tests/image_module_test.module
+++ b/modules/image/tests/image_module_test.module
@@ -9,6 +9,9 @@ function image_module_test_file_download($uri) {
   if (variable_get('image_module_test_file_download', FALSE) == $uri) {
     return array('X-Image-Owned-By' => 'image_module_test');
   }
+  if (variable_get('image_module_test_invalid_headers', FALSE) == $uri) {
+    return array('Content-Type' => 'image/png');
+  }
 }
 
 /**