MimeUtility.cs

using System;
using System.Collections.ObjectModel;
using System.IO;
using System.Linq;

namespace MimeMapping
{
    /// <summary>
    /// MIME type utility to lookup by file extension
    /// </summary>
    public static class MimeUtility
    {
        /// <summary>
        /// The "octet-stream" subtype is used to indicate that a body contains arbitrary binary data.
        /// See <a href="https://www.iana.org/assignments/media-types/application/octet-stream">application/octet-stream</a>
        /// </summary>
        public const string UnknownMimeType = "application/octet-stream";

        static Lazy<ReadOnlyDictionary<string, string>> _lazyDict = new Lazy<ReadOnlyDictionary<string, string>>(
            () => new ReadOnlyDictionary<string, string>(KnownMimeTypes.ALL_EXTS.Value.ToDictionary(e => e, e => KnownMimeTypes.LookupType(e)))
        );

        /// <summary>
        /// Dictionary of all available types (lazy loaded on first call)
        /// </summary>
        public static ReadOnlyDictionary<string, string> TypeMap => _lazyDict.Value;

        /// <param name="file">The file extensions (ex: "zip"), the file name, or file path</param>
        /// <returns>The mime type string, returns "application/octet-stream" if no known type was found</returns>
        public static string GetMimeMapping(string file)
        {
            if (file == null)
                throw new ArgumentNullException(nameof(file));

            if (string.IsNullOrEmpty(file))
                return UnknownMimeType;

            var fileExtension = file.Contains(".") 
                ? GetExtension(file)
                : file;

            return KnownMimeTypes.LookupType(fileExtension.ToLowerInvariant()) ?? UnknownMimeType;
        }

        /// <param name="mimeType">The mime type string, e.g. "application/json"</param>
        /// <returns>One or more extensions matching the mime type or null if no match</returns>
        public static string[] GetExtensions(string mimeType)
        {
            if (string.IsNullOrEmpty(mimeType)) throw new ArgumentNullException(mimeType);

            return KnownMimeTypes.LookupMimeType(mimeType);
        }

        private static string GetExtension(string path)
        {
            var extension = Path.GetExtension(path);
            if (string.IsNullOrEmpty(extension))
                return string.Empty; 

            return extension.Substring(1);
        }
    }
}