Populate file info for JSON files so that they can be uploaded to WordPress.

fix-json-uploads.php

<?php

/**
 * Populate file info for JSON files.
 *
 * @see https://core.trac.wordpress.org/ticket/45633
 *
 * @param array $info The existing file data array.
 * @param string $file Full path to the file.
 * @param string $filename The name of the file.
 * @param array $mimes Key is the file extension with value as the mime type.
 * @param string|bool $real_mime The actual mime type or false if the type cannot be determined.
 * @return array $info The modified file data array.
 */
function my_add_json_mime_types( $info, $file, $filename, $mimes, $real_mime ) {

    // Get filetype data.
    $wp_filetype = wp_check_filetype( $filename, $mimes );
    $ext = $wp_filetype['ext'];
    $type = $wp_filetype['type'];

    // Skip if not JSON.
    if ( $ext !== 'json' ) {
        return $info;
    }

    // Use finfo_file if available to validate non-image files.
    if ( empty( $real_mime ) AND function_exists( 'finfo_file' ) ) {
        $finfo = finfo_open( FILEINFO_MIME_TYPE );
        $real_mime = finfo_file( $finfo, $file );
        finfo_close( $finfo );
    }

    // If the extension matches an alternate mime type, let's use it.
    if ( in_array( $real_mime, ['application/json', 'text/plain', 'text/html'] ) ) {
        $info['ext'] = $ext;
        $info['type'] = $type;
    }

    // --<
    return $info;

}

// Add filter for the above.
add_filter( 'wp_check_filetype_and_ext', 'my_add_json_mime_types', 10, 5 );