Set the file extension when is not present in the URL

[mitogh]

When an image or a URL is downloaded using WordPress function download_url and if the provided URL does not contain the file extension, instead of using tmp try to guess the extension of the file by using the provided mime type of the file.

An important consideration is to make sure the mime type is supported by the current WordPress installation to prevent download and upload of non supported mime types on the current installation.

Trac ticket: https://core.trac.wordpress.org/ticket/54738

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