multi-part not working when setting content-type manually

[behrica]

I had a very strange issue, and it took me a long time to figure it out, so want to report it here.

When doing this request:

(def base-url "https://xxxxxx.westeurope.azurecontainerapps.io")
;(def base-url "http://localhost:5000")

(hc/post (format "/data-extraction?model_id=GPT-5.1" base-url)
              {
                 ;:headers {"Content-Type" "multipart/form-data"}
               :multipart [
                           {:name "prompt-template" :content prompt-template}
                           {:name "extraction-schema" :content (cheshire.core/encode json-schema)}
                           {:name "multiple-extractions" :content "false"}
                           {:name "pdfs" :content (io/file file)}
                           
                           ]

               :as :json})

things stop working, when I set additionally the "Content-Type" header to "multipart/form-data", and I saw in client side logging, that no form data is send.
But this does not happen, when the base-url is "http://localhost:5000"

I checked this 10 times, and it does depend on 'which' base URL is used.... I am user Docker so, client code is the same remote and local,

So with base-url being localhost, It does work with and without setting the content type header,
when using "https:/....." it only works "without setting content-tpy header by hand".
(which is very strange, as I assume that it gets set automatically in any case by hato)