When a multipart request (file upload) with the following content disposition header is received by Undertow, the resulting Part will have name "MyFile.txt" instead of "file":

Content-Disposition: form-data; filename="MyFile.txt"; name="file"

The reason for this seems to be that io.undertow.util.Headers#extractQuotedValueFromHeader is buggy and will use the value of the "filename" field if it is asked to extract the "name" field.

I tested this using the following piece of code:

import io.undertow.util.Headers;

class Test {
    public static void main(String[] args) {
        String res = Headers.extractQuotedValueFromHeader(
                "form-data; filename=\"myfile\"; name=\"file\"",
                "name");
        System.out.println(res);
    }
}

Running it will output "myfile" instead of the expected "file".

The Headers class is used during multipart extraction here:
https://github.com/undertow-io/undertow/blob/master/core/src/main/java/io/undertow/server/handlers/form/MultiPartParserDefinition.java#L239

A client that reproduces the given order of fields (filename first, then name) would be Jersey JAX-RS 2 with the Apache connector.