Could a potential cause of this bug be from this line:

This call to wp_match_mime_types() is not passing a mime type as the second function parameter.

No, I don't think that's cause. I don't think the code in and around that function is loaded on wp-admin/upload.php, which is where I'm seeing this happen.

I've done a little more checking in the code and that line I linked to matches upstream so probably not the cause of a problem.

I'm sure I've seen comments in the code that PHP is not 100% reliable at extracting mime type. Do you have an example file that creates this issue? Your proposed fix looks reasonable but I'd like to see if we can identify the cause earlier in the chain of events if possible.

A very recent thread on the inaccuracy of mime type detection is at https://forums.phpfreaks.com/topic/318512-problems-getting-mime-type-of-files/

I have quite a few SVGs that I have created from rasterized images using Inkscape. CP never recognizes their mime types. Here's one:

How are you enabling SVG upload to the Media Library? This is working locally for me, recognised as image/svg+xml, with the following code snippet to allow SVG uploads:

function allow_svg_upload( $mimes ) {
	$mimes['svg']  = 'image/svg+xml';
	$mimes['svgz'] = 'image/svg+xml';
	return $mimes;
}
add_filter( 'upload_mimes', 'allow_svg_upload' );

I'm using a plugin. It was Safe SVG before, now SVG Support.

I'm using a plugin. It was Safe SVG before, now SVG Support.

Strange one this, I've use my snippet and the SVG Support plugin and both allow upload of the SVG image you linked without anything logged in debug.log or shown on screen as an error. I'll try again next week.

I can't reproduce it either.