This PR was merged into the 4.x branch.

Discussion
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Supply default placeholder for file chooser

This fixes #5198

The problem was - with default (empty) placeholder `form_label` was falling back to use form type label (or filename when `currentFiles` was not empty). When provided with non-empty default value it is being used and it shows friendly "Choose file" placeholder.

Note: We cannot use default, browser placeholder, as the real file input is being hidden with `display: none`.

Commits
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dc4481c Supply default placeholder for file chooser