Licenses: Edit / New : Adding "Core document" and "Supplementary information" with uploads generates a system error

UPDATE: PLEASE READ THE NOTES AS IT LOOKS AS THOUGH THE ERROR IS MORE GENERALLY AROUND UPLOADING TWO DOCUMENTS AT ONCE (although not all the time!), REGARDLESS OF WHETHER OR NOT THEY BELONG TO A CORE OR SUPPLEMENTARY DOCUMENT. Apologies, I've spent quite a long time trying to tie down the circumstances which cause this one but haven't been able to, so the below description and video represent just one instance.

When editing / creating a license it is not possible to add an entry for "Core document" with an upload and "Supplementary information" with an upload at the same time. An error is generated. The two screen snips below show two errors which were generated on separate occasions.

The attached screencast shows how to generate one of the errors.

Steps to recreate:

1. Edit an existing license or create a new license.
2. Select "Add Core document", enter a name and upload a file
3. Select "Add supplementary document", enter a name and upload a file.
4. Save

Notes:

1. I have tested this with different documents, it just seems to be the presence of the file uploads and nothing to do with the files themselves.

2. If the documents are added and saved separately the error is not generated, only when they are added together and then saved.

3. I thought initially that the error was only generated when one Core and one Supplementary docs were added but I've just managed to generate one when adding two Supplementary Docs as well.

4. The issue exists for Amendments.as well - the below error was generated