Be skeptical of client-reported MIME Content-Types

Over the 14 years that I’ve been working on browsers and the web platform, I’ve seen a lot of bugs where the client’s configuration causes a problem with a website.

By default, Windows maintains File Extension to Content Type and Content Type to File Extension mappings mappings in the registry. You can find the former mappings in subkeys named for each file extension, e.g. HKEY_CLASSES_ROOT\.ext, and the latter as subkeys under the HKEY_CLASSES_ROOT\MIME\Database\Content Type key:


These mappings are how Internet Explorer, Edge, and other browsers know that a file delivered as Content-Type: application/pdf should be saved with a .pdf extension, and that a local file named example.html ought to be treated as Content-Type: text/html.

Unfortunately, these mappings are subject to manipulation by locally-installed software, which means you might find that installing Microsoft Excel causes your .CSV file upload to have a Content-Type of application/vnd.ms-excel instead of the text/csv your website was expecting.

Similarly, you might be surprised to discover that some popular file extensions do not have a MIME type registered by default on Windows. Perhaps the most popular of these is files in JavaScript Object Notation format; these generally should have the file extension .json and a MIME type of application/json but Windows treats these as an unknown type by default.

Today, I looked at a site which allows the user to upload a JSON file containing data exported from some other service. The upload process fails in Edge with an error saying that the file must be JSON. Looking at the script in the site, it contains the following:

validateFile = function(file) {
  if (file.type !== "application/json") // BUG BUG BUG
    { alert('That is not a valid json file.'); return; }

This function fails in Edge– the file.type attribute is the empty string because Windows has no mapping between .json and application/json.

This site usually works in Chrome because Chrome has a MIME-type determination system which first checks a fixed list of mappings, then, if no fixed mapping was found, consults the system registry, and finally, if the registry does not specify a MIME type for a given extension, Chrome consults a “fallback” list of mappings (kSecondaryMappings), and .JSON is in that final fallback list. However, even Chrome users would be broken if the file had the wrong extension (e.g. data.jso) or if the user’s registry contained a different mapping (e.g. .json=>”text/json”).

As a consequence, client JavaScript and server-side upload processing logic should be very skeptical of the MIME type contained in the file.type attribute or Content-Type header, as the MIME value reported could easily be incorrect by accident (or malice!).

-Eric Lawrence
PS: End users can workaround the problem with sites that expect particular MIME types for JSON by importing the following Registry Script (save the text as FixJSON.reg and double-click the file):

Windows Registry Editor Version 5.00

"Content Type"="application/json"

[HKEY_CLASSES_ROOT\MIME\Database\Content Type\application/json]



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