Great question — answering as the UphillTech team:
It's not a real film . It's a fictional 'classic' noir film made for this show. If you look them up on IMDb, the actors playing 'Leonard' and 'Velma' in that movie are contemporary actors and started acting in 2004 and 2006, respectively (and they don't play these characters anywhere else).
Full answer by Walt: https://movies.stackexchange.com/a/67207 (CC BY-SA 3.0).
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