I haven't found any configuration or guidance for adding Mullvad VPN to Obtainium with proper version tracking and that includes the community config currently provided at https://apps.obtainium.imranr.dev/
Using some basic regex I came up with a "match group" which seems to work in parsing out the version number on the"english" Mullvad VPN download page...
In Obtainium add the URL
https://mullvad.net/en/download/vpn/android
In the settings...
APK link filter:
latest$
Version String Extraction Regex:
version\: (\d\d\d\d.\d)
Match Group to Use:
$1
If someone has a better config or improved regex I'd be happy to hear it.