oEmbed system doesn't work if thumbnail url does not have a file extension

Problem/Motivation

After downloading a thumbnail for a remote oEmbed resource, certain operations (e.g., applying an image style to the thumbnail) can fail if the thumbnail had no file extension, with errors like:

Could not apply Medium (220×220) image style to public://oembed_thumbnails/5qPKeF_Fcn22-QFyRf6_CK8IX1YMBi4xeeLOjBrlWtY. because the style does not support it.

This has recently broken our support for Vimeo, which no longer supplies file extensions in its thumbnail URLs. As an example, see https://vimeo.com/api/oembed.json?url=https://vimeo.com/347119375. Note that the response has a thumbnail_url field, but it has no file extension.

Proposed resolution

If no file extension is present in the thumbnail URL, then use the MIME type of the thumbnail to add a file extension to the local copy of the thumbnail.

Remaining tasks

Review and commit the patch.

Release notes snippet

TBD