Explain things better

Author: javiereguiluz

reference/constraints/File.rst

@@ -245,9 +245,12 @@ You can find a list of existing mime types on the `IANA website`_.
 .. note::
 
     When using this constraint on a :doc:`FileType field </reference/forms/types/file>`,
-    the value of this option is also used in the ``accept`` attribute of the
-    related ``<input type="widget"/>`` HTML element (unless the field defines
-    its own ``accept`` value).
+    the value of the ``mimeTypes`` option is also used in the ``accept``
+    attribute of the related ``<input type="widget"/>`` HTML element.
+
+    This behavior is applied only when using :ref:`form type guessing <forms-field-guessing>`
+    (i.e. the form type is not defined explicitly in the ``->add()`` method of
+    the form builder) and when the field doesn't define its own ``accept`` value.
 
     .. versionadded:: 4.4