Issue Summary

Images referenced by relative attachment URLs in a custom PDF stylesheet are not rendered in Confluence Data Center 10.2.14.

Example relative URL:

background-image: url('/download/attachments/<content-id>/<filename>'); 

The equivalent absolute URL renders correctly:

background-image: url('https://<base-url>/download/attachments/<content-id>/<filename>'); 

A data-URL also renders correctly.

Steps to Reproduce

1. Create a Confluence space and page.
2. Attach a valid PNG file named pdf-test.png to the page.
3. Record the attachment download URL.
4. Configure this minimal space PDF stylesheet (replace URL with real attachment):

body {
    padding-top: 150px;
    color: #ff0000 !important;
    font-size: 24px !important;
    background-image:
        url('/download/attachments/<content-id>/pdf-test.png');
    background-repeat: no-repeat;
    background-position: left top;
} 

 6.  Perform a space PDF export or PDF export a single page in that space

Expected Results

The root-relative attachment URL should resolve to the same Confluence attachment as the equivalent same-base absolute URL.
Both of these forms should render the image:

url('/download/attachments/<content-id>/pdf-test.png')`
url('https://<base-url>/download/attachments/<content-id>/pdf-test.png') 

Actual Results

Only the absolute images render in exported PDF

Workaround

Use absolute URLs in PDF stylesheets