Hi there,
I’m not seeing any changes specifically that would cause it to stop working, and no discussion about FileBird on our side. Checking the changelog for the plugin, it looks like there were changes related to the media library back in March, however it looks like they were specific to WordPress.com sites:
https://www.ads-software.com/plugins/mailpoet/#developers
3.60.5 – 2021-03-22
Fixed: loading Media Library in Email Editor on WordPress.com sites;
However, I also see that FileBird isn’t one of the compatible plugins with a MailPoet integration either:
https://www.mailpoet.com/plugins/
https://kb.mailpoet.com/article/198-forms-themes-plugins-that-work-with-mailpoet
For now, if the FileBird plugin is no longer working, then it would likely require some custom code/integration to get working again.
It would be great if you could add that as a request on our feedback board here:
https://mailpoet.canny.io/