string|null guessClientExtension()
Returns the extension based on the client mime type.
If the mime type is unknown, returns null.
This method uses the mime type as guessed by getClientMimeType() to guess the file extension. As such, the extension returned by this method cannot be trusted.
For a trusted extension, use guessExtension() instead (which guesses the extension based on the guessed mime type for the file).
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