io.File seems like it should be the correct type.
The LinkedHashMap seems to be the one that’s incorrect… which was driving my thought that it was the JSON that wasn’t happy.
The format of your JSON doesn’t 100% match the docs that I linked to, and I can see how the REST API could get confused, thinking that the JSON block in the Value was the text to set the variable to.
"value": {
"type": "File",
"valueInfo": {
"filename": "pic.png",
"mimeType": "image/png"
},
"field1": "iVBORw0KGgoAAAANSUhEUgAABTAAAAJQCAYAAAC5Pku2AAAAAXNSR0IArs4c6QAAIABJREFUeF7s3Qm8TeX7//8rocxEkjmSimRsIqIoKT5IRSIlQ6XJECmlNItSCJUMiTIUUSKJSoWiKInMKmTI0Mz/8V7f39n/"
}
as compared to
"value": "iVBORw0KGgoAAAANSUhEUgAABTAAAAJQCAYAAAC5Pku2AAAAAXNSR0IArs4c6QAAIABJREFUeF7s3Qm8TeX7//8rocxEkjmSimRsIqIoKT5IRSIlQ6XJECmlNItSCJUMiTIUUSKJSoWiKInMKmTI0Mz/8V7f39n/",