Simple dictionary provides a few methods to lookup mime type/extension, generated from Apache's mime.types.
via NuGet:
PM> Install-Package MimeTypesMap
using HeyRed.Mime;
MimeTypesMap.GetExtension("image/jpeg"); // => jpeg
MimeTypesMap.GetMimeType("filename.jpeg"); // => image/jpeg
MimeTypesMap.AddOrUpdate(string mime, string extension);