Extract attachment from content-type "multipart/mixed"

Hello,

Structure of pipeline:
REST POST → mapper outputs binary view → multipart reader

REST POST’ response has content-type:
“content-type”:[{
“value”:
“multipart/mixed”
“value_1”:
" \tboundary="----=text for boundary""
}]
response also has a zip attachment.

mapper takes document view and outputs response with binary view.

Multipart reader:
Content-Type : $.headers[‘content-type’]
provided content type from the response.

The multipart reader fails:
Failure: class com.google.common.collect.AbstractMapBasedMultimap$RandomAccessWrappedList cannot be cast to class java.lang.String (com.google.common.collect.AbstractMapBasedMultimap$RandomAccessWrappedList is in unnamed module of loader org.eclipse.jetty.webapp.WebAppClassLoader @4e282e97; java.lang.String is in module java.base of loader ‘bootstrap’), Reason: Snap failed unexpectedly and did not provide any reason

Feel free to point out my mistakes.

Thanks for your time,
Smit