Found a way. I had to implement a coercion-matcher function coercing a yada.multipart.DefaultPart to a java.io.File. I extended the yada.multipart/default-part-coercion-matcher to add {java.io.File my-coercing-fn}, this coercing function take the part, transform the :bytes byte array to a ByteArrayInputStream, skip n bytes (where n is the part's :body-offset) and then use to copy the stream content to a file in /tmp/. So of course for large files it could be problematic in term of memory consumption and disk space, but this fit my use case ATM.