PDF attached with mimetype HTML - breaks opening attached file in composition window

$ lsb_release -rd
Description: Ubuntu Jammy Jellyfish (development branch)
Release: 22.04

$ apt-cache policy thunderbird
thunderbird:
  Installed: 1:91.8.0+build2-0ubuntu1
  Candidate: 1:91.8.0+build2-0ubuntu1
  Version table:
 *** 1:91.8.0+build2-0ubuntu1 500
        500 http://de.archive.ubuntu.com/ubuntu jammy/main amd64 Packages
        100 /var/lib/dpkg/status
     1:91.8.0+build2-0ubuntu0.18.04.1 500
        500 http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu bionic/main amd64 Packages
        500 https://ppa.launchpadcontent.net/ubuntu-mozilla-security/ppa/ubuntu bionic/main amd64 Packages

3) I composed an email, attached a pdf, tried to open by double-click - then I expected it to be opened as PDF in any PDF viewer.

4) I composed an email, attached a pdf (either drag and drop or through the file picker dialog) and when double-clicking to open it the dialog claims it was filetype HTML (so the app choice breaks).

I checked the .thunderbird/<profile.default>/mimeTypes.rdf but can't find anything surprising there.

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: thunderbird 1:91.8.0+build2-0ubuntu1
ProcVersionSignature: Ubuntu 5.15.0-25.25-generic 5.15.30
Uname: Linux 5.15.0-25-generic x86_64
ApportVersion: 2.20.11-0ubuntu80
Architecture: amd64
BuildID: 20220401203107
CasperMD5CheckResult: pass
CurrentDesktop: MATE
Date: Thu Apr 14 20:04:09 2022
InstallationDate: Installed on 2022-03-07 (38 days ago)
InstallationMedia: Ubuntu 22.04 LTS "Jammy Jellyfish" - Alpha amd64 (20220228)
SourcePackage: thunderbird
UpgradeStatus: No upgrade log present (probably fresh install)