Attachment upload to custom content in Confluence not working in Chrome

Summary

Using Google Chrome, attaching a custom content is not working and being silently canceled.

Steps to Reproduce

1. Using Google Chrome (Version 67.0.3396.87)
2. Edit custom content in iframe
3. Using AP.request, from within the add-ons iframe upload an image as attachment via PUT to '/wiki/rest/api/content/{id}/child/attachment' endpoint with options

   {
      contentType: 'multipart/form-data', 
      processData: false, 
      headers: {
         "X-Atlassian-Token":"nocheck"
      }, 
      cache: false
   } 
   

Expected Results

Attachment is uploaded.

Actual Results

The request was canceled.

Notes

• Works with other browsers.
• The feature of uploading images has been shipped on May 12th and it did work initially. So I guess that it stopped working at some point in May or at the beginning of June.