[Mozilla Enterprise] Format of inline IconURL for SearchEngines in policies.json ?

[James Pearson]

I'm trying to add an IconURL for a custom Search Engine in a 
policies.json file for ESR 60 - but I'm not sure of the format/syntax

I have an existing Search Plugin XML file that has the syntax for the 
Icon of the form (which works):

<Image width="16" height="16">data:image/png;base64,iVBORw0...YII=</Image>

I've tried using the policies.json syntax:

  "IconURL": "data:image/png;base64,iVBORw0...YII=",

but I just get a generic icon for my Search Engine

The Policy Engine docs don't give any info on what the syntax should be 
- except stating: "URL to icon" ...

Anyone know what the correct syntax should be?

Thanks

James Pearson