Hi Sean and welcome to our forums,
The MIME type limitation from Advanced Installer is actually a reflection of the limitation which was imposed to us by Windows Installer.
To handle the extensions installation we use standard MSI table, as we do for other parts of the MSI package too, in this case we use the MIME table
As you can see in Microsoft docs the column "ContentType" is an identifier so it must be unique for each row from the table. Usually this type of limitation is easily overpassed using public properties, but according to MSDN this column is not formatted, so unfortunately you cannot use this trick.
Also, my main executable has two embedded icons. When I try to map two different ProgIDs to the second embedded icon, I get this error message:
"A row with this key [c:\somepath\my.exe#1] was already registered in the table"
And then Advanced Installer crashes. Anyway to reuse the second icon with multiple ProfIDs?
I tried reproducing this in a new project, but with no success. Can you reproduce it in a new project?