Using Spring WebClient to upload a file as inputstream

Im using a webflux springboot application. where i have an API which takes a FilePart as input and then convert it to inputstream since i make a filetype check using Tika library and then upload this file to API using webclient i get 500 internal server error from the server

MultipartBodyBuilder builder = new MultipartBodyBuilder();
        builder.part("file", new InputStreamResource(inputStream)).header("Content-Disposition",
            "form-data; name=file; filename="test.txt");
return webClient
            .post()
            .uri("https://example.com/file")
            .contentType(MediaType.MULTIPART_FORM_DATA)
            .body(BodyInserters.fromMultipartData(multipartBody))
            .retrieve()
            .bodyToMono(String.class);

but if i skip converting the file to input stream the webclient works

return webClient.post()
                .uri("https://example.com/file")
                .contentType(MediaType.MULTIPART_FORM_DATA)
                .body(BodyInserters.fromMultipartData("file", filePart))
                .retrieve()
                .bodyToMono(String.class);

the internal server error you are getting is not actually related to WebClient, but you have two other issues in your code:

1. String Concatenation in the Content-Disposition header, because of the double quotes around text.txt.
2. second is that you didn't build the MultiPartBodyBuilder, and multipartBody used in BodyInserters.fromMultipartData(multipartBody) is not initialized.

the WORKING code:

MultipartBodyBuilder builder = new MultipartBodyBuilder();
builder.part("file", new InputStreamResource(inputStream))
   .header("Content-Disposition", "form-data; name=\"file\"; 
filename=\"test.txt\"");

//building MultipartBodyBuilder and assigning it to multipartBody
MultiValueMap<String, HttpEntity<?>> multipartBody = builder.build();

return webClient`enter code here`
    .post()
    .uri("https://example.com/file")
    .contentType(MediaType.MULTIPART_FORM_DATA)
    .body(BodyInserters.fromMultipartData(multipartBody))
    .retrieve()
    .bodyToMono(String.class);

notice the use of \" instead of " in around file and test.txt in Content-Disposition header String.