Following is an example:

URL: {{url}}/api/rest/issues/123/notes
METHOD: POST
PAYLOAD:

{
  "text": "This is a test note with two attachment",
  "view_state": {
    "name": "public"
  },
  “files”: [
    {
        "name": "test1.txt",
        "content": "VGhpcyBpcyBhIFRFU1QuDQpUaGlzIGlzIGEgVEVTVC4NClRoaXMgaXMgYSBURVNULg0KVGhpcyBpcyBhIFRFU1QuDQpUaGlzIGlzIGEgVEVTVC4="
    },
    {
        "name": "test2.txt",
        "content": "VGhpcyBpcyBhIFRFU1QuDQpUaGlzIGlzIGEgVEVTVC4NClRoaXMgaXMgYSBURVNULg0KVGhpcyBpcyBhIFRFU1QuDQpUaGlzIGlzIGEgVEVTVC4="
    }
  ]
}