Perceived types

Windows classifies file extensions into broad kinds, the perceived types: text, image, audio, video and so on. File Explorer uses them, for instance, to choose the columns of a folder of pictures. Perceived from the Meziantou.Framework.Win32.PerceivedType package returns the perceived type of a file name or an extension.

dotnet package add Meziantou.Framework.Win32.PerceivedType

Get the perceived type of a file

Pass a file name or an extension:

// Get perceived type for a file extension
var perceived = Perceived.GetPerceivedType(".txt");
Assert.Equal(PerceivedType.Text, perceived.PerceivedType);
Assert.NotEqual(PerceivedTypeSource.Undefined, perceived.PerceivedTypeSource); // e.g. SoftCoded, NativeSupport

// Works with various file types
var image = Perceived.GetPerceivedType(".jpg");
Assert.Equal(PerceivedType.Image, image.PerceivedType);

var video = Perceived.GetPerceivedType(".mp4");
Assert.Equal(PerceivedType.Video, video.PerceivedType);

var audio = Perceived.GetPerceivedType(".mp3");
Assert.Equal(PerceivedType.Audio, audio.PerceivedType);

The result gives the Extension, the PerceivedType, and the PerceivedTypeSource, which tells how Windows knows the type:

The types are Text, Image, Audio, Video, Compressed, Document, System, Application, GameMedia and Contacts. An extension that Windows does not know is Unspecified, and Custom is a type defined in the registry that is not one of the others.

The answer depends on the machine: Windows and the installed applications register the file associations, so the same extension can have a different type, or none, on another machine. An extension with no registered association is Unspecified.

Define the perceived type of an extension

Define the types your application relies on, instead of depending on the applications installed on the machine:

// Add a custom perceived type
Perceived.AddPerceived(".myext", PerceivedType.Text);
Assert.Equal(PerceivedType.Text, Perceived.GetPerceivedType("file.myext").PerceivedType);

// Add default perceived types for common extensions
Perceived.AddDefaultPerceivedTypes();

• AddPerceived replaces the type of an extension for the current process only: it does not change the registry.
• AddDefaultPerceivedTypes defines common development extensions: .cs, .csproj, .sln, .xaml, .html, .js, .css, .sql and others as Text, and .exe, .dll and .class as Application.
• The extensions are compared without regard to case.

Caching

GetPerceivedType remembers the answers of Windows for the lifetime of the process, so a change of the file associations is not seen until the application restarts. The cache is bounded: when the names come from untrusted input, such as uploaded files, unusual extensions do not make it grow without limit.