Here's a Mulligan file:
{
"GameCardId":"Mulligan",
"ConfigComment":"These are the Mulligan rules.",
"Mulligan":
{
"values":
[
{
"comment": "Only hold Prince Keleseth (ICC_851)",
"mulligan":"ICC_851",
"condition": "*",
"value":"hold",
},
{
"comment": "Discard all cards not listed as hold",
"mulligan":"*",
"condition": "*",
"value":"discard",
}
],
},
}