images not appearing in export zip (BB-11504)

When I insert an image inline in an issue description or comment (ie by drag and drop into the image insert popup), the image displays in the field just fine. However, when I export the issues, those image files aren't included in the export ZIP file.

The exported JSON for the description/comment containing the image includes the markdown that includes the image, like

![1523 test_noedit0.png](https://bitbucket.org/repo/pn5XgL/images/2038502625-1523%20test_noedit0.png)

but the actual image file is not in the ZIP file.