I tried the “open with” suggestion that AMCarter3 posted on October 14 (“I tried a different method to open LibreOffice and it worked perfectly… just hold the Control Key down then you click on the app to open it, and click on OK to acknowledge you accept responsibility for opening an app that the OS cannot identify.”). This not totally necessary because there was no “app” option. Then I discovered that the original “error message” contains 3 options, “cancel” and “open”. Hit “open” and LO opens! Problem solved. Ignore the error message.
I assume that you are double clicking on the FileZilla install icon, yes? Don’t do that. Instead right mouse click on it. You should get an Open option in context menu: Select Open. Then the next dialog box you see will be similar to the one you report, but it will also have the Open button added. Choose Open and voila.
It’s just a general tip to fix app cannot open in macOS Catalina issue. Hope it may help you.