Adds 'enctype="multipart/form-data"' to the form

[lloc]

This PR adds the enctype="multipart/form-data" attribute to the site creation form on /wp-admin/network/site-new.php.

Related ticket: https://core.trac.wordpress.org/ticket/62086

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props realloc. 

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[github-actions]

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[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 60398
GitHub commit: fb4ec6f

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.