After years of research, I finally found how to do that with my Xojo version: “à l’ancienne” .
First of all, I will disclose how I do it for json files and it worked fine (even if the file is an sqlite3 file internally):
Display name: JSON Document (JSON)
Object name: JSON
Icon: a custom icon was provided.
Yes, I know, some properties above are deprecated.
In the macOS File Types, I set Editor for that file type.
Firefox have an icon for json files, but my custom icon is used.
BETTER: when I change the file extension (of a .sqlite file), the json icon is immediately set / removed when I removed the .json file extension. So far, so good.
During years, here and then, I tried to change my .sqlite file definition to no available.
Minutes ago, I found my solution:
MacType and MacCreator set to ???
UTIs: nothing (empty).
File Extensions: .sqlite
macOS File Type set to Editor
Open the Builds folder…
SURPRISE: I get my custom icon.
How do I achieve that (think to do that) ?
The solution came to me when I was thinking: “I’ve done everything.”
Then: “No, I do not let blank some parts”.
So I removed the UTIs, PROCESS…
I fill MacType and MacCreator with ???, PROCESS…
…and it worked.
Do not “Ask Me Why” this works.