The definitive way to add more filetypes to WordPress Media.

class-uploads.php

<?php
/**
 * Filters that assist in uploading additional filetypes.
 *
 * @package    Your_Package
 * @subpackage Your_Package/Includes
 */

namespace Your_Package\Includes;

/**
 * Central spot to manage the uploads.
 */
class Uploads {

	/**
	 * Constructor.
	 */
	public function __construct() {
		add_filter( 'upload_mimes', [ $this, 'filter_upload_mimes' ] );
		add_filter( 'site_option_upload_filetypes', [ $this, 'filter_site_option_upload_filetypes' ] );
		add_filter( 'wp_check_filetype_and_ext', [ $this, 'determine_proper_filetype_and_ext' ], 9, 5 );
	}

	/**
	 * Array of allowed mime types keyed by the file extension.
	 * The first in the list is used when any of the mimes are found for that extension.
	 *
	 * @var array[]
	 */
	static $mimes = [
		'geojson' => [ 'application/json', 'text/json' ],
		'json'    => [ 'application/json', 'text/json' ],
		'kml'     => [ 'application/vnd.google-earth.kml+xml', 'application/xml', 'text/xml' ],
		'csv'     => [ 'text/csv', 'text/plain', 'application/csv', 'text/comma-separated-values' ],
	];

	/**
	 * Filter: upload_mimes; add additional filetypes to the allowed upload filetypes.
	 *
	 * @param string[] $mimes Array of allowed mime types keyed by the file extension.
	 *
	 * @return mixed
	 */
	public function filter_upload_mimes( $mimes ) {
		$additional_mimes = self::$mimes;

		// Reduce the array to only the first mime type.
		$additional_mimes = array_map( 'reset', $additional_mimes );

		// Merge the additional mimes with the existing mimes.
		$mimes = array_merge( $mimes, $additional_mimes );

		return $mimes;
	}

	/**
	 * Filter: site_option_upload_filetypes; add additional filetypes to the allowed upload filetypes.
	 *
	 * @param string $option_value Space separated list of allowed filetypes (extensions).
	 *
	 * @return string
	 */
	public function filter_site_option_upload_filetypes( $option_value ) {
		$option_value = explode( ' ', $option_value );
		$option_value = array_merge( $option_value, array_keys( self::$mimes ) );
		$option_value = array_unique( $option_value );
		$option_value = implode( ' ', $option_value );

		return $option_value;
	}

	/**
	 * Filter: wp_check_filetype_and_ext; determine the proper filetype based on extension and a list of allowed mime types.
	 * WordPress only allows one filetype per extension, this filter implementation allows us to support multiple filetypes per extension.
	 *
	 * @param array    $file_data An array of data for a single file, as determined by WordPress during upload.
	 * @param string   $file      Unused in this implementation. The path to the uploaded file.
	 * @param string   $filename  The name of the uploaded file.
	 * @param string[] $mimes     Unused in this implementation.
	 * @param string   $real_mime The mime-type as determined by PHP's Fileinfo extension.
	 *
	 * @return mixed
	 */
	public function determine_proper_filetype_and_ext( $file_data, $file, $filename, $mimes, $real_mime ) {
		$file_ext = pathinfo( $filename, PATHINFO_EXTENSION );
		foreach ( self::$mimes as $ext => $mime ) {
			if ( $ext === $file_ext && in_array( $real_mime, self::$mimes[ $ext ], true ) ) {
				$file_data['ext']  = $ext;
				$file_data['type'] = reset( self::$mimes[ $ext ] );
			}
		}

		return $file_data;
	}
}

new Uploads();