CTaskItem::deleteFile

public function deleteFile($attachmentId)
{
	Attachment::delete($this->getId(), $attachmentId, array(
		'USER_ID' => $GLOBALS['USER']->GetId(),
		'ENTITY_ID' => UserField::getTargetEntityId(),
		'FIELD_NAME' => 'UF_TASK_WEBDAV_FILES'
	));

	// drop cache
	$this->markCacheAsDirty();
}