Getting a MIME type from a file extension in ASP.NET 4.5

Getting a MIME type from a file extension in ASP.NET 4.5

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If you've ever implemented a file upload or download scenario in ASP.NET, you've likely come across the situation of having to provide a MIME type based off of a file extension. IIS does this automatically when it serves files, but it seems like in ASP.NET there isn't a good way to provide this functionality. Searches on the web might point you to building your own Dictionary of MIME types or just writing a switch statement, but this felt like a hack to me and so I turned to a resident expert, Levi Broderick.

It turns out in ASP.NET 4.5 we shipped a little-known new type, System.Web.MimeMapping, which has an API called GetMimeMapping(string fileName). Here is the MSDN documentation. You can pass either a file name as the method implies, or an extension.

This is awesome. It turns what would have been a pain into a one line call. But how is it implemented - did the ASP.NET team just do the "hack" for you by building a table?

The answer is yes and no. If you are running IIS in classic mode, the fallback is indeed a Dictionary of over 300 mappings of file extensions to MIME types. However, if you are running in integrated pipeline mode (most people), the list of MIME types used by the API is actually the list in IIS. This means that as you upgrade Windows and more MIME types are added to IIS, the code you write doesn't need to be updated to take advantage of the changes.

Hope this hidden gem helps!

Update: As folks have mentioned in the comments, this should not be the only check you run on a file upload, but it does simply the development step of generating the MIME type mapping once you have verified that you have the correct file type by checking bytes, for security reasons.

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  • You should not rely on MIME anymore than you rely on file extension. The proper way is to read the magic bytes of the file to make sure they are actually what you expect. Failing that you could open the door to security vulnerabilities. Only do this to filter the magic bytes list you need to compare your file to, not as the only file type check.

  • Initially, I was quite positive about the thought of having a MIME mapping API in the base class libraries; it appeared to be a Microsoft supported solution to a common requirement found in many ASP.NET / HTTP based projects. However, upon inspecting the members of the API, I can't believe Microsoft used this opportunety to introduce such a simple and naive implementation.

    Only a single method (public static string GetMimeMapping(string fileName)) - that's it. No sign of enumerable members or e.g. TryGet(string fileName, out string mapping). What's the risk in allowing 3rd party developers to enumerate the discovered (static as well as dynamic) mime mappings by provide e.g. a simple key/value-pair enumerator?

    Definitely not up to BCL standards.

  • Hey folks, thanks for the feedback - Arnaud, you are correct that this is only the first step in making sure you have a secure file upload endpoint - I'll update the post to add that point so folks aren't misled.

    @Anders: That's a good question/suggestion - I can follow up with the team perhaps to get more background on why that decision was made.

  • Thanks for sharing this valuable information. :)

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