How to Get MIME Types from File Extensions in .NET Core

It’s a fairly common issue – you need to get the MIME type for a given file extension. Maybe you need to set this properly for an email attachment, or it’s being set as part of uploading to Azure Blob Storage (note: weird stuff can happen if you don’t set the MIME type properly for Azure Blob Storage!).

In .NET Framework (4.5+) you could’ve used the MimeMapping.GetMimeMapping() method, but this isn’t part of .Net Core.

A Native .Net Core Solution

.Net Core introduced the class FileExtensionContentTypeProvider, which is basically a wrapper around a mapping between file extension and content (MIME) type.

It can be used as follows:

using Microsoft.AspNetCore.StaticFiles;

public string GetMimeTypeForFileExtension(string filePath)
{
    const string DefaultContentType = "application/octet-stream";

    var provider = new FileExtensionContentTypeProvider();

    if (!provider.TryGetContentType(filePath, out string contentType))
    {
    contentType = DefaultContentType;
    }

    return contentType;
}

Adding Custom MIME Type Mappings

The in-built mappings may not support all the file extensions you need. Fortunately, you can add additional mappings like so:

var provider = new FileExtensionContentTypeProvider();

if (!provider.Mappings.TryGetValue(".csv", out var _))
{
    // Add new MIME mapping
    provider.Mappings.Add(".csv", "text/csv");
}
else
{ 
    // Replace existing MIME mapping as it already exists
    provider.Mappings[".csv"] = "text/csv";
}

Note that the ‘.’ needs to prefix any file extensions added.

Bonus: Let’s Make this an Extension Method

There are not many methods on FileExtensionContentTypeProvider – it seems to jut be a thin wrapper around the dictionary of mappings. In the interests of creating a richer API, here’s an extension method to add or update a mapping inspired by the code above:

using Microsoft.AspNetCore.StaticFiles;
...

public static void AddOrUpdateMapping(
    this FileExtensionContentTypeProvider provider,
    string fileExtension,
    string contentType)
{
    if (!provider.Mappings.TryGetValue(fileExtension, out var _))
    {
       provider.Mappings.Add(fileExtension,contentType);
    }
    else
    { 
       provider.Mappings[fileExtension] = contentType;
    }
}

Summary

When looking up MIME types in .Net Core, consider using the FileExtensionContentTypeProvider class.