Using REST API it is not possible to add attachment to Paragraph(Multi-Line) Custom field.

• Paragraph Custom field need id in this format. 

-d '{
    "fields": {
      "customfield_10126": {
        "type": "doc",
        "version": 1,
        "content": [
          {
            "type": "mediaGroup",
            "content": [
              {
                "type": "media",
                "attrs": {
                  "type": "file",
                  "id": "0ded4f84-15ab-4de1-bc77-71a5d5c5b23e",
                  "collection": ""
                }
              }
            ]
          }
        ]
      }
    }' 

• However attachment id which we uploaded using rest/api/3/issue/issueIdOrKey/attachments is different.

  "attachment": [
        {
          "self": "https://site.atlassian.net/rest/api/3/attachment/10067",
          "id": "10067",
          "filename": "2.jpg",
          "author": {
  
  

• There is no way to identify which media id is mapped with attachment id.

Expected Scenario

There should be a REST API available to add attachment easily to Paragraph custom field or there should be a REST API which can provide mapping of attachment id with media id. Then that media id can be used with edit issue API to add attachment to custom-field.