Add a POST file to the upload
string $field POST field to use (e.g. file). Used to reference content from the server.:
string $filename Full path to the file. Do not include the @ symbol.:
string $contentType Optional Content-Type to add to the Content-Disposition.: Default behavior is to guess. Set to false to not specify.
EntityEnclosingRequestInterface
public function addPostFile($field, $filename = null, $contentType = null);