Is there a set of constants in the framework for all the standard content types?
Here is the complete list currently available in .NET Framework class library (aka base class library, BCL) System.Net.Http.dll:  
System.Net.Mime.MediaTypeNames.Application.Octet for application/octet-streamSystem.Net.Mime.MediaTypeNames.Application.Pdf for application/pdfSystem.Net.Mime.MediaTypeNames.Application.Rtf for application/rtfSystem.Net.Mime.MediaTypeNames.Application.Soap for application/soap+xml System.Net.Mime.MediaTypeNames.Application.Zip for application/zip System.Net.Mime.MediaTypeNames.Image.Gif for image/gifSystem.Net.Mime.MediaTypeNames.Image.Jpeg for image/jpeg System.Net.Mime.MediaTypeNames.Image.Tiff for image/tiff System.Net.Mime.MediaTypeNames.Text.Html for text/htmlSystem.Net.Mime.MediaTypeNames.Text.Plain for text/plain System.Net.Mime.MediaTypeNames.Text.RichText for text/richtextSystem.Net.Mime.MediaTypeNames.Text.Xml for text/xml