A filter (an mu-plugin) to restore CSV upload functionality to WordPress 4.9.9 and up.

wp-499-up-fix-csv-upload.php

<?php
/**
 * Restore CSV upload functionality for WordPress 4.9.9 and up
 */
add_filter('wp_check_filetype_and_ext', function($values, $file, $filename, $mimes) {
	if ( extension_loaded( 'fileinfo' ) ) {
		// with the php-extension, a CSV file is issues type text/plain so we fix that back to 
		// text/csv by trusting the file extension.
		$finfo     = finfo_open( FILEINFO_MIME_TYPE );
		$real_mime = finfo_file( $finfo, $file );
		finfo_close( $finfo );

		if ( $real_mime === 'text/plain' && preg_match( '/\.(csv)$/i', $filename ) ) {
			$values['ext']  = 'csv';
			$values['type'] = 'text/csv';
		}
	} else {
		// without the php-extension, we probably don't have the issue at all, but just to be sure...
		if ( preg_match( '/\.(csv)$/i', $filename ) ) {
			$values['ext']  = 'csv';
			$values['type'] = 'text/csv';
		}
	}

	return $values;
}, PHP_INT_MAX, 4);

Thanks!

I had the same issue but the CSV being imported had some HTML in it. I assume that is the reason why the "$real_mime" was "text/html", and this fix didn't work. Strangely we've used dozens of CSV files before and only recently had this become an issue.

I changed line 13 to include: $real_mime === 'text/plain' || $real_mime === 'text/html'