SplFileObject::getMimeType()

FileObject.php

<?php

class FileObject extends \SplFileObject
{
    private $size;

    public function fwrite($content, $len = null) // :int
    {
        $this->size = $len ?: strlen($content);

        if (!$len) {
            return parent::fwrite($content);
        }

        return parent::fwrite($content, $len);
    }

    public function getMimeType() // :string
    {
        $finfo = new finfo(FILEINFO_MIME);

        $this->fseek(0);
        return $finfo->buffer(
            $this->fread($this->size),
            FILEINFO_MIME_TYPE
        );
    }
}

index.php

<?php

$f  = file_get_contents('https://pbs.twimg.com/profile_images/655066410087940096/QSUlrrlm.png', false, null, -1, 1024);
$fo = new FileObject("php://temp/file.tmp", "w+");

$fo->fwrite($f);

var_dump($fo->getMimeType());