Add info about files behavior (#4422)

* Add info about files behavior

* Update articles/components/upload/index.adoc

Co-authored-by: Jouni Koivuviita <jouni@vaadin.com>

* Update articles/components/upload/index.adoc

Co-authored-by: Jouni Koivuviita <jouni@vaadin.com>

* Update articles/components/upload/index.adoc

Co-authored-by: Jouni Koivuviita <jouni@vaadin.com>

---------

Co-authored-by: Jouni Koivuviita <jouni@vaadin.com>

Author: TatuLund

articles/components/upload/index.adoc

@@ -567,6 +567,11 @@ include::{root}/frontend/demo/component/upload/react/upload-error-messages.tsx[r
 endif::[]
 --
 
+=== Component State
+
+To conserve resources, the files themselves aren't part of the component's server-side state. This has significant implications for component behavior. Removing the Upload component from the layout means that the files in the browser memory are lost. The same applies when reloading the page. Read more about <</components/upload/file-handling#, file handling>>.
+
+The user interface, including the Upload component, should take this into account. For example, if you expect that the user will refresh the page, you can display a message to the user that files needs to be selected again, if this wasn't the first time the page was rendered.
 
 == Related Components