How do I select a certificate file when enabling SAML single logout

On the Edit SAML Integration page of my web app, I’ve enabled Single Logout, and can find and select the certificate file that applies via both Browse and Upload Certificate buttons, but only the filename appears in the input field, and when I click Next, I get the error: “The filename is invalid or missing”.

I must be missing something, but I don’t know what that might be. I’d like to enable Single Logout for completeness, Single Sign On is working just fine, the SAML validation (including the security tests) has passed, and I’m almost ready to submitting the app for review.