public HttpRequest bodyForm(final Parameters formParams) {
        this.requestBodySupplier = () -> {
            final FormBody.Builder entity = new FormBody.Builder();
            if (formParams != null) {
                for (final ImmutablePair<String, String> param : formParams) {
                    entity.add(param.left, param.right);
                }
            }
            return entity.build();
        };
        return this;
    }
Sets body to Multipart-FORM, including one optional File body part.
Set "file" to null to not pass a file. If "file" is not null, then "fileMediaType" must be set as well
main/src/main/java/cgeo/geocaching/network/HttpRequest.java