Reimbursements: investigate attached invoices

[pkevan]

There are some unusual behaviours attached to the reimbursement workflow that requires investigation, some details can be found in this Slack thread: https://wordpress.slack.com/archives/C08M59V3P/p1715872397770189.

Notably -

• if a user is unable to attach a file to a post, and appropriate error message should be displayed.
• super admins (or arguably admins) should be able to attach any file from the media library, given they should have access to all file that might be restricted via the privacy plugin see: https://github.com/WordPress/wordcamp.org/blob/production/public_html/wp-content/plugins/wordcamp-payments/includes/privacy.php#L18-L35.