Change default file upload size limit in salesforce

You'll want to read Inputs and Binary File Uploads, but basically, you can't use base-64-encoded data, as far as I can tell. You could try setting Content-Transfer-Encoding to BASE64, but I'm not sure that's supported. That would look like this:

--boundary-code-1234567
Content-Disposition: ...
Content-Type: ...
Content-Transfer-Encoding: BASE64

base64ContentHere===
----boundary-code-1234567

If that doesn't work, then presumably you'll need to use binary instead. Most languages include a way to post literal binary data to a data stream, so it shouldn't be too bad. However, I've never had a need to do this, so you may need to experiment a bit.

I hope this answer helps clarify what's going on.

Partner support have replied to me and provided a working solution!

Follow this tutorial https://developer.salesforce.com/blogs/developer-relations/2017/05/build-lightning-file-uploader-component.html but change the file upload apex class to 'without sharing'. Then embed the component within a flow following from the record creation passing through the new record id into the recordId attribute (I did it through a design attribute).

For now this will upload to notes & attachments rather than files but I assume a process builder or other solution could move it over if required.

Partner support does note that: ">>If you want to try this, another thing to keep in mind is the entry point of the Apex without sharing logic should be protected by some secure access code, for ex. an encrypted record id. "