File Thumbnails or Hyperlinks

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "a",
  "txtContent": "[$FileRef]",
  "attributes": {
    "target": "_blank",
    "href": "=@currentWeb + '/_layouts/15/getpreview.ashx?resolution=3&path=' + [$FileRef]"
  }
}

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/column-formatting.schema.json",
  "elmType": "a",
  "txtContent": "[$FileRef]",
  "attributes": {
    "target": "_blank",
    "href": {
      "operator": "+",
      "operands": [
        "@currentWeb",
        "/_layouts/15/getpreview.ashx?resolution=3&path=",
        "[$FileRef]"
      ]
    }
  }
}

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "div",
  "children": [
    {
      "elmType": "div",
      "style": {
        "width": "64px",
        "height": "64px",
        "overflow": "hidden",
        "border-radius": "10%"
      },
      "children": [
        {
          "elmType": "img",
          "attributes": {
            "src": "=@currentWeb + '/_layouts/15/getpreview.ashx?resolution=1&path=' + [$FileRef]"
          },
          "style": {
            "position": "relative",
            "top": "50%",
            "left": "50%",
            "width": "100%",
            "height": "auto",
            "margin-left": "-50%",
            "margin-top": "-50%"
          }
        }
      ]
    }
  ]
}