public MultipartEntityBuilder setBoundary(final String boundary) {
        this.boundary = boundary;
        return this;
    }
Sets a custom boundary string for the multipart entity.
<p>
If {@code null} is provided, the builder reverts to its default logic of using a random UUID.
</p>
<p>
IMPORTANT: when setting an explicit boundary, it is responsibility of the caller to validate / sanitize content
of body parts to ensure they do not contain the boundary value.
</p>

@param boundary the boundary string, or {@code null} to use a random UUID.
@return this builder instance
httpclient5/src/main/java/org/apache/hc/client5/http/entity/mime/MultipartEntityBuilder.java