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);

You can create an mu-plugin file with this content, or place it in your theme functions.php, whatever floats your boat :)

I'm still getting the same error for some reason. I tested with and without the closing bracket. Could you provide some Guidance? Much appreciated as I know this should be much simpler. I installed the MU plugin successfully but still no results even though I see the plugin live even on the back end. We're just using a standard CSV utility for importing Products and products updates overnight. It is still failing. File type error.

@Linuxhombre

Yep it did work for me in WP 5.0.1 using WooCommerce 3.5.2.

"We're just using a standard CSV utility for importing Products and products updates overnight"
Sorry, this fix is for uploading CSV files to the Media library, I'm not sure why your task fails, but if I understand you correctly, this is not a Media Library problem. I probably could help you out, but I would need much more detailed information, error logs, etc. Access to your system (FTP) would help. I am for hire :) (but due to the Christmas & New Years rush that might be second half of January)

There is a different solution, maybe, probably. If your troubles started with WordPress 4.9.9 or higher, and you had no problem with 4.9.8, just revert. Easiest way to do this is by using the command line, but that would require

• SSH access to your website hosting (a good hosting company allows this, so if not, time to shop around)
• WP-CLI software present (should be no problems at all once you have SSH access)
  Then using the wp-cli software, from inside the WordPress directory;
  (this command is elaborate on purpose so it works for everybody, not just for those with the WP-CLI properly set-up)

php path/to/your/copy/of/wp-cli.phar core update --force --version=4.9.8

You should probably disable auto-updates as well, Google is your friend.

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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'