When we are adding a group of contacts to Event from found Contacts list A non-recoverable error is occurring:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] =>
[message] => HTML_QuickForm_Controller: Unhandled action "upload" in page "Participant"
[mode] => 16
[debug_info] =>
[type] => PEAR_Error
[user_info] =>
[to_string] => [pear_error: message="HTML_QuickForm_Controller: Unhandled action "upload" in page "Participant"" code=0 mode=callback callback=CRM_Core_Error::handle prefix="" info=""]
)