ImageService: avoid possible NPE by calling equals() on literal instead of on variable.

Reported by PMD.

No functional changes.

Author: php-coder

src/main/java/ru/mystamps/web/service/ImageService.java

@@ -50,7 +50,7 @@ String save(final MultipartFile file) {
 
 		final String extension = StringUtils.substringAfter(contentType, "/");
 		checkState(
-			extension.equals("png") || extension.equals("jpeg"),
+			"png".equals(extension) || "jpeg".equals(extension),
 			"File type must be PNG or JPEG image"
 		);