Looks like it's never getting downloaded from the server:
<div class="content-container">
<h3>HTTP Error 404.3 - Not Found</h3>
<h4>The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.</h4>
</div>
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<fieldset><h4>Most likely causes:</h4>
<ul> <li>It is possible that a handler mapping is missing. By default, the static file handler processes all content.</li> <li>The feature you are trying to use may not be installed.</li> <li>The appropriate MIME map is not enabled for the Web site or application. (Warning: Do not create a MIME map for content that users should not download, such as .ASPX pages or .config files.)</li> <li>If ASP.NET is not installed.</li> </ul>
</fieldset>
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<fieldset><h4>Things you can try:</h4>
<ul> <li>In system.webServer/handlers: <ul> <li>Ensure that the expected handler for the current page is mapped.</li> <li>Pay extra attention to preconditions (for example, runtimeVersion, pipelineMode, bitness) and compare them to the settings for your application pool.</li> <li>Pay extra attention to typographical errors in the expected handler line.</li> </ul> </li> <li>Please verify that the feature you are trying to use is installed.</li> <li> Verify that the MIME map is enabled or add the MIME map for the Web site using the command-line tool appcmd.exe. <ol> <li>To set a MIME type, run the following command in the IIS Express install directory: <b>appcmd set config /section:staticContent /+[fileExtension='string',mimeType='string']</b></li> <li>The variable fileExtension string is the file name extension and the variable mimeType string is the file type description.</li> <li>For example, to add a MIME map for a file which has the extension ".xyz": <b>appcmd set config /section:staticContent /+[fileExtension='.xyz',mimeType='text/plain']</b></li> </ol> Warning: Ensure that this MIME mapping is needed for your Web server before adding it to the list. Configuration files such as .CONFIG or dynamic scripting pages such as .ASP or .ASPX, should not be downloaded directly and should always be processed through a handler. Other files such as database files or those used to store configuration, like .XML or .MDF, are sometimes used to store configuration information. Determine if clients can download these file types before enabling them. </li> <li>Install ASP.NET.</li> <li>Check the failed request tracing logs for additional information about this error. For more information, click <a href="http://go.microsoft.com/fwlink/?LinkID=66439">here</a>. </li> </ul>
</fieldset>
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<h4>More Information:</h4>
This error occurs when the file extension of the requested URL is for a MIME type that is not configured on the server. You can add a MIME type for the file extension for files that are not dynamic scripting pages, database, or configuration files. Process those file types using a handler. You should not allows direct downloads of dynamic scripting pages, database or configuration files.
Host
localhost:65303
User-Agent
Mozilla/5.0 (Windows NT 10.0; …) Gecko/20100101 Firefox/56.0
Accept
model/gltf-binary,model/gltf+j…ication/json;q=0.2,*/*;q=0.01
Accept-Language
en-US,en;q=0.5
Accept-Encoding
gzip, deflate
Referer
http://localhost:65303/Demo/Hello3DWorld
Cookie
__RequestVerificationToken=OnE…IcxbVx0GU89g4Ou0ey_iPuf-YGT9w
Connection
keep-alive
So server configuration now has:
<httpCompression directory="%TEMP%\iisexpress\IIS Temporary Compressed Files">
<scheme name="gzip" dll="%IIS_BIN%\gzip.dll" />
<dynamicTypes>
<add mimeType="text/*" enabled="true" />
<add mimeType="message/*" enabled="true" />
<add mimeType="application/javascript" enabled="true" />
<add mimeType="application/atom+xml" enabled="true" />
<add mimeType="application/xaml+xml" enabled="true" />
<add mimeType="model/gltf+json" enabled="true" />
<add mimeType="*/*" enabled="true" />
</dynamicTypes>
<staticTypes>
<add mimeType="text/*" enabled="true" />
<add mimeType="message/*" enabled="true" />
<add mimeType="image/svg+xml" enabled="true" />
<add mimeType="application/javascript" enabled="true" />
<add mimeType="application/atom+xml" enabled="true" />
<add mimeType="application/xaml+xml" enabled="true" />
<add mimeType="model/gltf+json" enabled="true" />
<add mimeType="*/*" enabled="true" />
</staticTypes>
</httpCompression>
<staticContent lockAttributes="isDocFooterFileName">
<mimeMap fileExtension=".gltf" mimeType="model/gltf+json" />
</staticContent>
and
<urlCompression doDynamicCompression="true" />
Has anyone successfully served gltf file from IIS Express?