Consumes two multipart/form-data fields:

1. metadata – JSON body EncryptStreamRequest
   
2. file – binary plaintext
   
   
   Produces a multipart/mixed response where:
   
   • Part 1 (application/jose+json) is a JWE General-JSON header with
   detached ciphertext.
   
   • Part 2 (application/octet-stream) is the proprietary envelope
   [wkLen]‖wrappedKey‖[ivLen]‖iv‖ciphertext‖tag streamed on-the-fly.
   
   
   Output format depends on key type:
   
   • SIMPLE keys: JWE General JSON with single recipient
   
   • COMPOSITE keys: JWE General JSON with multiple recipients (one per component)