Reimplemented original addToOffset method.

Uploader/ErrorHandler/BlueimpErrorHandler.php

@@ -16,6 +16,6 @@ public function addException(ResponseInterface $response, UploadException $excep
             $message = $exception->getMessage();
         }
 
-        $response->addToOffset('files', array('error' => $message));
+        $response->addToOffset(array('error' => $message), 'files');
     }
 }

Uploader/Response/AbstractResponse.php

@@ -33,12 +33,34 @@ public function offsetGet($offset)
         return isset($this->data[$offset]) ? $this->data[$offset] : null;
     }
 
-    public function addToOffset($offset, array $value)
+    /**
+     * The \ArrayAccess interface does not support multi-dimensional array syntax such as $array["foo"][] = bar
+     * This function will take a path of arrays and add a new element to it, creating the path if needed.
+     *
+     * @param mixed $value
+     * @param mixed $offset,...
+     *
+     * @throws \InvalidArgumentException if the path contains non-array or unset items.
+     */
+    public function addToOffset($value, $offset)
     {
-        if (!array_key_exists($offset, $this->data)) {
-            $this->data[$offset] = array();
-        }
+        $args = func_get_args();
+        array_shift($args);
+
+        $element =& $this->data;
 
-        $this->data[$offset] += $value;
+        foreach ($args as $offset) {
+            if (isset($element[$offset])) {
+                if (is_array($element[$offset])) {
+                    $element =& $element[$offset];
+                } else {
+                    throw new \InvalidArgumentException('The specified offset is set but is not an array at ' . $offset);
+                }
+            } else {
+                $element[$offset] = array();
+                $element =& $element[$offset];
+            }
+        }
+        $element[] = $value;
     }
 }