In trying to add HTML5 Application Cache support to one of my web apps today, I hit a few little snags with my hoster (GoDaddy with a Shared Deluxe Windows account) and wanted to share as I’m sure other people will run into the same thing.

Each time I uploaded my web.config file, I kept getting 500 server errors that weren’t very helpful.  As it turns out, the extension .manifest is already taken by the mime-type application/x-ms-manifest, and when I tried to override that, IIS got kind of angry, and served the 500 Server error.  I wasn’t quite sure what was happening, but was able to figure it out by reverting to my original web.config file, and requesting the appcache.manifest file that I had already uploaded.  Sure enough, using the network tab in the Chrome DevTools, I saw it was being served back with the wrong mime type.

To resolve the issue, I added a new static file handler for .appcache files in the system.web -> httpHandlers section.  If you don’t do this, IIS doesn’t know about the file type and it won’t serve unknown file types at all.  Then in system.webServer -> staticContent, I added a mimeMap extension for .appcache files with the mimetype of text/cache-manifest.  After uploading the web.config file again, I retried my request, and sure enough, everything worked perfectly!

My final web.config now looks like…

<configuration>
  <system.web>
    ...
    <httpHandlers>
      ...
      <add verb="GET,HEAD" path="*.appcache"
        type="System.Web.StaticFileHandler" />
    </httpHandlers>
  </system.web>
  ...
  <system.webServer>
  ...
    <staticContent>
    <mimeMap fileExtension=".appcache"
      mimeType="text/cache-manifest" />
    ...
    </staticContent>
    ...
  </system.webServer>
  ...
</config>

Check out this tutorial on Application Cache at HTML5Rocks.com. Also, you can find a full list of the default mime types provided by GoDaddy’s IIS servers here.

[Update 6/30/11 @ 1:53pm] @Paul_Irish pointed out that the recommended extension is .appcache specifically to avoid the unregistered Microsoft extension, and referred to a bug on HTML5.org. So there you have it! :)