Returns an immutable instance of [MultipartField].
public MultipartField<T> build()Returns an immutable instance of [MultipartField]. Further updates to this [Builder] will not mutate the returned instance. The following fields are required:
.value()If [contentType] is unset, then it will be set to application/octet-stream if [value]
is binary data, or text/plain; charset=utf-8 otherwise.
IllegalStateException: if any required field is unset.