This looks very interesting. Alas, it doesn't pass security on Mac OS Mojave.
You should be able to right-click on the application icon and choose "open" from the context menu that pops up to make this problem go away. (Assuming you trust the developer, of course.) This message is just a more strongly worded (and less convenient) version of the dialog that has always come up when opening side-loaded apps for the first time, at least in my experience.
It looks like Apple's newly introduced "notarization" process will avoid this step, even for apps distributed outside of the official App Store, but that process will take a little bit of time. In the meantime you'll need to take the right-click-and-choose-open action.
The version now available at https://www.fatpick.com/downloads/FATpick-mac-latest.dmg is fully notarized by Apple and should not have this problem.
Release notes here: https://www.fatpick.com/blog/release-notes-fatpick-v1-4-2
Would love to try it!
Are you seeing that problem when opening the DMG or the app itself? Does it help to use the right-click-and-choose-open approach?
For what it is worth you can download (a zipped version of) the .app file directly from https://www.fatpick.com/releases/FATpick-1.3.5-mac.zip, if that would work around the problem.
It is not yet notarized, but only because of my own ignorance of the implications of that. (In my defense the notarization thing is a relatively new policy.)
xcrun altool --notarize-app --primary-bundle-id "com.your.bundleid" --username "username" --password "password" --file "/path/to/installbundle"
Wait for apple to approve it.
Then staple it.
[1] https://developer.apple.com/documentation/security/notarizin...
In the meantime you can make an explicit exception by "right-clicking" on the application icon and selecting "Open" from the context menu that pops up.
The version currently available at https://www.fatpick.com/downloads/FATpick-mac-latest.dmg is fully notarized by Apple and should not have this problem.
Release notes here: https://www.fatpick.com/blog/release-notes-fatpick-v1-4-2
Please let me know if you encounter any problems with this new build. You can reach me via the contact form at https://www.fatpick.com/contact-us or you can email me directly at rod@<that-domain>.