[Form] added multiple option to file type doc

Author: bamarni

reference/forms/types/file.rst

@@ -9,6 +9,8 @@ The ``file`` type represents a file input in your form.
 +-------------+---------------------------------------------------------------------+
 | Rendered as | ``input`` ``file`` field                                            |
 +-------------+---------------------------------------------------------------------+
+| Options     | - `multiple`_                                                        |
++-------------+---------------------------------------------------------------------+
 | Inherited   | - `required`_                                                       |
 | options     | - `label`_                                                          |
 |             | - `read_only`_                                                      |
@@ -76,6 +78,16 @@ before using it directly.
 Read the :doc:`cookbook </cookbook/doctrine/file_uploads>` for an example of
 how to manage a file upload associated with a Doctrine entity.
 
+Field Options
+-------------
+
+multiple
+~~~~~~~~
+
+**type**: ``Boolean`` **default**: ``false``
+
+When set to true, the user will be able to upload multiple files at the same time.
+
 Inherited options
 -----------------