Discourage local-FS-only methods

Magento2/Sniffs/Functions/DiscouragedFunctionSniff.md

@@ -0,0 +1,11 @@
+# Rule: getimagesize() is discouraged
+
+## Reason
+
+[getimagesize](https://www.php.net/manual/en/function.getimagesize.php) function works only with local and supported streams. 
+With introduction of more advanced storages, like AWS S3 or Azure Blob Storage this function will cause issues where file is not accessible.
+
+## How to fix
+
+[getimagesizefromstring](https://www.php.net/manual/en/function.getimagesizefromstring.php) can be used instead to retrieve all the information from file.
+This function works with data strings, so you should read the file content using specific adapter before using it.

Magento2/Sniffs/Functions/DiscouragedFunctionSniff.php

@@ -228,5 +228,6 @@ class DiscouragedFunctionSniff extends ForbiddenFunctionsSniff
         '^intval$' => '(int) construction',
         '^strval$' => '(string) construction',
         '^htmlspecialchars$' => '\Magento\Framework\Escaper->escapeHtml',
+        '^getimagesize$' => 'getimagesizefromstring',
     ];
 }