Define Web Font Mime Types within the Web.config of an ASP.NET Project to ensure that the Web Server properly handles serving up your Fonts.

Web.xml

  <system.webServer>
    <staticContent>
      <!-- Remove any potential Web Server Settings for Web Fonts to be used -->
      <remove fileExtension=".eot" />
      <remove fileExtension=".otf" />
      <remove fileExtension=".ttf" />
      <remove fileExtension=".woff" />
      <remove fileExtension=".woff2" />
      
      <!-- Define Web Font Mime Types that will be used -->
      <mimeMap fileExtension=".eot" mimeType="application/vnd.ms-fontobject" />
      <mimeMap fileExtension=".otf" mimeType="application/font-sfnt" />
      <mimeMap fileExtension=".ttf" mimeType="application/font-sfnt" />
      <mimeMap fileExtension=".woff" mimeType="application/font-woff" />
      <mimeMap fileExtension=".woff2" mimeType="font/woff2" />
      
      <!-- Define SVG Mime Type (could be used for Images or Fonts) -->
      <remove fileExtension=".svg" />
      <mimeMap fileExtension=".svg" mimeType="image/svg+xml" />
    </staticContent>
  </system.webServer>

application/font-sfnt is not the right mimetype for ttf or otf - not sure why you would use it?

Try application/x-font-ttf or application/x-font-truetype for ttf and application/x-font-opentype for otf

Edit: I can see some history on it here: https://stackoverflow.com/a/20723357 hope this helps others! <3