metadata.php: add sniff_image

inc/functions/metadata.php

@@ -34,3 +34,26 @@ function ext_to_mime(string $ext): ?string {
 	];
 	return $ext_to_mime[$ext] ?? null;
 }
+
+/**
+ * Sniffs the image content.
+ *
+ * @param string $file_path The path to an image file.
+ * @return array An array with the image's width, height and mime type.
+ * @throws \RuntimeException On error, or on non-image files.
+ */
+function sniff_image(string $file_path): array {
+	$ret = \getimagesize($file_path);
+	if ($ret === false) {
+		throw new \RuntimeException("Could not read image sizes of '$file_path'");
+	}
+	if ($ret[2] == \IMAGETYPE_UNKNOWN) {
+		throw new \RuntimeException("Error '$file_path' is not an image");
+	}
+
+	$width = $ret[0];
+	$height = $ret[1];
+	$mime = $ret['mime'];
+
+	return [ $width, $height, $mime ];
+}