Issue Summary

Custom uploaded images in headers are not exported through XML/CSV

This is reproducible: (yes) 

Steps to Reproduce

1. Create a page on Confluence Cloud
2. Click on Add image header and add an image from your local computer 
3. Publish the page
4. Navigate to Space Settings -> Export Space -> XML -> Full Export
5. Download the export file and import it on another site.
6. Navigate to the page where the header was added and notice the header missing

Expected Results

The header image should be imported and displayed on the page.

Actual Results

The header image is not displayed on the imported page.

Workaround

Currently, there is no known workaround for this behaviour. A workaround will be added here when available