Since support for PDF previews were added in [38949], it's possible
that the generated image file could overwrite an existing image file
with the same name. This uses `wp_unique_filename()` to avoid this
issue and adds a '-pdf' identifier on the end of filenames.
Props gitlost, derosj, mikeschroder, joemcgill.
Fixes#39875. See #31050.
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