Adobe Sign REST API v5: Uploading File to File Attachment Field

I'm using the Adobe Sign REST API v5's POST /workflow/{workflowId}/agreements.

I'm trying to upload an attachment to a file attachment field. However, I can't seem to get it to pre-fill. Do you have any suggestions or possibly see the mistake I'm doing?

Sample Body:

{ documentCreationInfo: {
fileInfo: [{ name: "document", workflowLibraryDocumentId: "id"}],
mergeFieldInfo: [{ fieldName: "file.attachment", default: "data:image/png;base64, ..."}],
name: "name",
recipientsListInfo: [ { name: "Signer1",  recipients: [{ email: "email@hotmail.com"}] }]
}

Alternatively, I tried importing as a file:

{ documentCreationInfo: {
fileInfo: [
{ name: "document1", workflowLibraryDocumentId: "id"}, 
{ name: "document2", documentURL: {  url: "data:image/png;base64, ..." }
],
mergeFieldInfo: [],
name: "name",
recipientsListInfo: [ { name: "Signer1",  recipients: [{ email: "email@hotmail.com"}] }]
}