400 error code on uploading attachment on creating new page

Summary

Getting 400 error code on uploading attachment while creating new page, Insert button disabled

Environment:

Application :Confluence 6.10.2 (Build: 7702)
Operating System Windows Server 2016
Database Microsoft SQL Server 2014, Postgres

Steps to Reproduce

• Create new page
• DO NOT SAVE
• Click the Insert Files and Images from the Macro Browser
• Use the Upload File button to attempt to upload attachments

Expected Results

• To get insert button available when file uploaded successfully

Actual Results

The below exception is thrown on HAR file

Request URL: http://localhost:26110/c6110/rest/api/content/1835020?expand=version,container,space&_=1537358989931
Request Method: GET
Status Code: 400 

• By inspecting Response

  {"statusCode":400,"data":{"authorized":false,"valid":true,"allowedInReadOnlyMode":true,"errors":[],"successful":false},"message":"The entity Draft = { id: 1835019, type: page, title: null, pageId: 0, pageVersion: 0} is not ContentConvertible or API available","reason":"Bad Request"}
  

Workaround

• Save new page first then upload documents.