magento/magento2#11252: fix adminhtml file attribute edit form

[Mkennethsmith]

Applying fix for issue 11252 to Magento2 version 2.2.2
Backport of #11267

Description

This fixes issue #11252 file not allowing uploads for custom customer attributes in the Admin environment. FileUploaderTest passes with implemented change.

Fixed Issues (if relevant)

1. Custom attribute - File not allowing uploads #11252: Custom attribute - File not allowing uploads

Manual testing scenarios

1. Create a Customer custom attribute of type file
2. Attempt to upload a file in the admin panel.
3. Run Unit test app/code/Magento/Customer/Test/Unit/Model/FileUploaderTest.php

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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1 out of 2 committers have signed the CLA.

✅ Mkennethsmith
❌ mustdobetter

[orlangur]

Nice catch @Mkennethsmith! This is a backport of https://github.com/magento/magento2/pull/11267/files.

[magento-engcom-team]

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