links to attached PDF broken when using space to HTML export

Problem summary

When exporting space/page containing PDF file to HTML format, links to PDF attachments are not set correctly.

Environment

Confluence 6.3 Linux 64 bit, Chrome 55.0.2883.95

Replication steps

1. Create new space
2. Within new space create new page
3. Edit page and add PDF file by using drag & drop feature
4. Go to Space tools -> Content -> export. Select HTML then click next.
5. Select Normal export (custom export is affected as well)
6. Download exported HTML file
7. Unzip the file
8. Open index.html in any browser.
9. Go to page containing PDF attachments

Replication steps recording:

• Available at http://recordit.co/LYxs6PWqjm

Exptected result

• Attachment would open upon clicking on it.
• PDF thumbnail should be visible in a same way as on Confluence page

Actual results

• PDF image thumbanil not available
• Link from thumbnail is not opening
• Exported page contains "Attachments" section which contains working links to attached PDF file

Workaround

unknown