[Issue #19247] fix php 7.2 error : Countable interface not implemented

[gmachure]

Description (*)

We created a custom module with ui component form with a file upload field. While the upload,the method _setUploadFileId from vendor/magento/framework/File/Uploader.php generate a warning because Countable interface is not implement for a string (PHP version : 7.2.12).
$file[0] is a string and not an array. So count($file[0]) generate an error.

Fixed Issues (if relevant)

1. Magento 2 : fix php 7.2 error : Countable interface not implemented #19247: fix php 7.2 error : Countable interface not implemented

Manual testing scenarios (*)

1. Upload image in ui component form in a custom module
2. Test :
   -Create custom module
   -Add a file uploader in ui component form.
   -Upload an image.
   -PHP version : 7.2.12

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

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[magento-engcom-team]

Hi @gmachure. Thank you for your contribution
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