I recently upgraded to MacOS Catalina, and had the same problem. The issue is that Catalina has very strict file permissions. I was forced to go to System Preferences -> Security & Privacy -> Full Disk Access
From there I added /System/Library/CoreServices/Jar Launcher.app
My Mac is now less secure, but at least Hero Designer can find my characters!
Hero Designer does not recognize any file type on my Mac
in HERO Designer
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I recently upgraded to MacOS Catalina, and had the same problem. The issue is that Catalina has very strict file permissions. I was forced to go to System Preferences -> Security & Privacy -> Full Disk Access
From there I added /System/Library/CoreServices/Jar Launcher.app
My Mac is now less secure, but at least Hero Designer can find my characters!