Fix config backend model PDF file name extension

[Vinai]

Description (*)

The PDF system config file backend model does not include the file name ending pdf in the list of allowed extensions.
Instead, it allows the extensions ['tif', 'tiff', 'png', 'jpg', 'jpe', 'jpeg'], which are not relevant for PDF files.

This PR adds pdf to the list of allowed extensions without removing the existing wrong extensions. This way it's a backward compatible change, and it makes the backend model usable for PDFs.

It also adds a new backend model \Magento\Config\Model\Config\Backend\Image\Pdf as a replacement that only returns the allowed file name extension pdf.

I added a @deprecated annotation to the existing backend model with an explanation and a @see annotation to point to the new PDF backend model.

Manual testing scenarios (*)

Referencing the backend model in a system.xml file, e.g

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="foo" showInDefault="1" showInWebsite="1" showInStore="1" sortOrder="200">
            <label>Foo</label>
            <tab>general</tab>
            <resource>Magento_Customer::config_customer</resource>
            <group id="bar" showInDefault="1" showInWebsite="1" showInStore="1" sortOrder="10">
                <label>Example</label>
                <field id="baz" type="file" showInDefault="1" showInWebsite="1" showInStore="1" sortOrder="10">
                    <label>PDF Upload</label>
                    <backend_model>Magento\Config\Model\Config\Backend\Image\Pdf</backend_model>
                    <upload_dir scope_info="1">foo/bar</upload_dir>
                    <base_url type="media" scope_info="1">foo/bar</base_url>
                </field>
            </group>
        </section>
    </system>
</config>

Then upload a file with a .pdf extension. It will be rejected by the server side validation in \Magento\Config\Model\Config\Backend\File::beforeSave

Questions or comments

I'm not sure what the best way is to get this PR merged as quickly as possible.
If the deprecation of the @api class and addition of a new @api class prevent it from being merged, I would suggest to remove those annotations and just merge the file changes, and then add the annotations in a new PR that can be merged in the next major release.
Open to other suggestions if there is a better approach.

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 are green)

[m2-assistant]

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[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-5648 has been created to process this Pull Request
✳️ @sidolov, could you please add one of the following labels to the Pull Request?

Label	Description
Auto-Tests: Covered	All changes in Pull Request is covered by auto-tests
Auto-Tests: Not Covered	Changes in Pull Request requires coverage by auto-tests
Auto-Tests: Not Required	Changes in Pull Request does not require coverage by auto-tests

[engcom-Delta]

✔️ QA passed

[engcom-Foxtrot]

@magento run all tests

[Vinai]

Thanks for catching the namespace bug. Regarding the @inheritdoc annotation, according to https://devdocs.magento.com/guides/v2.3/coding-standards/docblock-standard-general.html#inheritdoc it should not be used.
I guess it's okay if it's still required to get the tests to pass, but I hope it won't be necessary anymore soon.

[Vinai]

@sidolov Is there anything I can do to help move this PR forward?

[m2-assistant]

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