I assigned a filetype with an Icon to a sqlite file.
I set it as Editor somewhere in the project (This Computer / MacOS).
The file extension is .sqlite.
As UTI, I used public.database (it was: public.database.sqlite.
I even open Get Info for one of the files and set my application as the default owner (so when I double click the document, it is open with my application).
I shutdown the computer, wait a but while drink a coffee, poser it ON, open the folder where my document files resides and these docments still have the undefined Icon.
Did I forgot something ?
Edit: I forgot to say that I set all Icons sizes and masks.