Implement File Block - I

[etoledom]

gutenberg PR: WordPress/gutenberg#26278
gutenberg-mobile PR: wordpress-mobile/gutenberg-mobile#2743

This is a first step on implementing File Block.

This implementation only considers the basics:

• Insert and render a File Block.
• Upload a file from the iOS native Documents Picker.
  • Don't let uploading file types not allowed by the blog.
• Edit the file name and Download button text.

The rest of features will be implemented in future PRs

EDIT: PICK A FILE has been changed to CHOOSE A FILE.

To test:

• Add a File block.
• Upload a file from the device.
  • Check that you are able to upload images, videos, audio, pdf... but not allowed to upload txt files.
• Check that you can edit the file name and the Download button text.
• Check on HTML mode that the HREF is the remote one (not local).
• Check on preview that you can download the uploaded file.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[jkmassel]

We're cutting 16.1 today, so this PR is being bumped to the 16.2 release. If you need it to be part of the 16.1 series, please merge it into release/16.1 and DM me – I'll be happy to create a new beta release!

[guarani]

Approved via WordPress/gutenberg#26278 (review)