I am trying to launch a callout call to an OCR from an Apex method using a secured named credential with an external credential.

The HTTP request is a POST request with multipart body containing a file.
I am using this response to [compose the multipart body][1]. 

Here a code sample I am using

        ContentDocument document = [SELECT LatestPublishedVersion.VersionData FROM ContentDocument WHERE Id = 'XXXXX'];
        Blob fileContent = document.LatestPublishedVersion.VersionData;
        String contentType = vNHttpFormBuilder.GetContentType();
        String form64 = '';
        form64 += vNHttpFormBuilder.WriteBoundary();
        vNHttpFormBuilder.WriteFileResult result = vNHttpFormBuilder.WriteFile('file', 'test File Name.pdf', 'pdf', fileContent);
        form64 += result.Content;
        form64 += vNHttpFormBuilder.WriteBoundary(result.EndingType);
        Blob formBlob = EncodingUtil.base64Decode(form64);
        String contentLength = String.valueOf(formBlob.size());
        HttpRequest req = new HttpRequest();
        req.setEndpoint('callout:TestCredential/api/publicapi/v1/transactions/launch?skillId=123');
        req.setMethod('POST');
        req.setHeader('Content-Type', contentType);
        req.setHeader('Connection', 'keep-alive');
        req.setHeader('Content-Length', contentLength);
        req.setTimeout(20000);

        req.setBodyAsBlob(formBlob);
        Http http = new Http();
        HttpResponse response = http.send(req);


But I keep getting this error when making the call. Origin of the error is the ```http.send``` method.
[![enter image description here][2]][2]

Even though the call seems to work just fine when replacing the endpoint with.
```req.setEndpoint('https://vantage-eu.abbyy.com/api/publicapi/v1/transactions/launch?skillId=123');```

PS: I have tried the named credential (the http call with 'callout:TestCredential') with a GET endpoint and it works fine.

I am suspecting that the script executed by Salesforce to inject the authentication token is failing due to some error with the blob body.


  [1]: https://salesforce.stackexchange.com/questions/132135/how-can-i-compose-a-multipart-form-data-request
  [2]: https://i.sstatic.net/UxYEP.png