fix wysiwyg editor not decoding base64 filenames special chars

[adammada]

Description

Properly decode base64 special chars in TinyMCE. Whole part of code was changed in 2.3, but it is not working in 2.2

Fixed Issues (if relevant)

1. WYSIWYG editor fails to parse directives of files with special characters in URL (so random files) #18138: WYSIWYG editor fails to parse directives of files with special characters in URL (so random files)

Manual testing scenarios

1. WYSIWYG editor fails to parse directives of files with special characters in URL (so random files) #18138

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @adammada. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me $VERSION instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3054 has been created to process this Pull Request

[magento-engcom-team]

@adammada thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[magento-engcom-team]

Hi @adammada. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.