Server side merge in Ruby

Which product are you using?
PDF.js Express Plus

PDF.js Express Version

Detailed description of issue
We are trying to implement XFDF merging on the server side in a Ruby on Rails application. All requests result in error 20, Invalid Input, and we always pass the file in URL form.
What is the exact request payload like? Can someone please provide a CURL example?

Expected behaviour
XFDF annotations should be merged.

Does your issue happen with every document, or just one?
All documents we tried, including known working ones.

Link to document

Code snippet
Request is multipart using Faraday.

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Hey there,

Can you please provide a code snippet showing how you are sending the request? If you could also log the payload that would be useful.