Pimcore\Model\Asset\Image\Thumbnail::getMimeType PHP Method

    public function getMimeType()
    {
        if (!$this->mimetype) {
            // get target mime type without actually generating the thumbnail (deferred)
            $mapping = ["png" => "image/png", "jpg" => "image/jpeg", "jpeg" => "image/jpeg", "pjpeg" => "image/jpeg", "gif" => "image/gif", "tiff" => "image/tiff", "svg" => "image/svg+xml"];
            $targetFormat = strtolower($this->getConfig()->getFormat());
            $format = $targetFormat;
            $fileExt = \Pimcore\File::getFileExtension($this->getAsset()->getFilename());
            if ($targetFormat == "source" || empty($targetFormat)) {
                $format = Thumbnail\Processor::getAllowedFormat($fileExt, ["jpeg", "gif", "png"], "png");
            } elseif ($targetFormat == "print") {
                $format = Thumbnail\Processor::getAllowedFormat($fileExt, ["svg", "jpeg", "png", "tiff"], "png");
                if (($format == "tiff" || $format == "svg") && \Pimcore\Tool::isFrontentRequestByAdmin()) {
                    // return a webformat in admin -> tiff cannot be displayed in browser
                    $format = "png";
                }
            }
            if (array_key_exists($format, $mapping)) {
                $this->mimetype = $mapping[$format];
            } else {
                // unknown
                $this->mimetype = "application/octet-stream";
            }
        }
        return $this->mimetype;
    }