Proper MIME type for OTF fonts

Question

Searching the web, I find heaps of different suggestions for what the proper MIME type for a font is, but I have yet to try any MIME type that rids me of a Chrome warning such as the following:

Resource interpreted as font but transferred with MIME type font/otf

The font is an OTF.

I've tried the following MIME types so far

• font/otf
• application/font-otf
• application/font
• application/otf
• application/octet-stream
• application/x-font-otf
• application/x-font-TrueType (I know it's not truetype, but one source quoted this for OTF)

Answer 1

There are a number of font formats that one can set MIME types for, on both Apache and IIS servers. I've traditionally had luck with the following:

svg   as "image/svg+xml"                  (W3C: August 2011)
ttf   as "application/x-font-ttf"         (IANA: March 2013)
      or "application/x-font-truetype"
otf   as "application/x-font-opentype"    (IANA: March 2013)
woff  as "application/font-woff"          (IANA: January 2013)
woff2 as "application/font-woff2"         (W3C W./E.Draft: May 2014/March 2016)
eot   as "application/vnd.ms-fontobject"  (IANA: December 2005)
sfnt  as "application/font-sfnt"          (IANA: March 2013) 

According to the Internet Engineering Task Force who maintain the initial document regarding Multipurpose Internet Mail Extensions (MIME types) here: http://tools.ietf.org/html/rfc2045#section-5 ... it says in specifics:

"It is expected that additions to the larger set of supported types can generally be accomplished by the creation of new subtypes of these initial types. In the future, more top-level types may be defined only by a standards-track extension to this standard. If another top-level type is to be used for any reason, it must be given a name starting with "X-" to indicate its non-standard status and to avoid a potential conflict with a future official name."

As it were, and over time, additional MIME types get added as standards are created and accepted, therefor we see examples of vendor specific MIME types such as vnd.ms-fontobject and the like.

UPDATE August 16, 2013: WOFF was formally registered at IANA on January 3, 2013 and Webkit has been updated on March 5, 2013 and browsers that are sourcing this update in their latest versions will start issuing warnings about the server MIME types with the old x-font-woff declaration. Since the warnings are only annoying I would recommend switching to the approved MIME type right away. In an ideal world, the warnings will resolve themselves in time.

UPDATE February 26, 2015: WOFF2 is now in the W3C Editor's Draft with the proposed mime-type. It should likely be submitted to IANA in the next year (possibly by end of 2016) following more recent progress timelines. As well SFNT, the scalable/spline container font format used in the backbone table reference of Google Web Fonts with their sfntly java library and is already registered as a mime type with IANA and could be added to this list as well dependent on individual need.

UPDATE October 4, 2017: We can follow the progression of the WOFF2 format here with a majority of modern browsers supporting the format successfully. As well, we can follow the IETF's "font" Top-Level Media Type request for comments (RFC) tracker and document regarding the latest set of proposed font types for approval.

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For those wishing to embed the typeface in the proper order in your CSS please visit this article. But again, I've had luck with the following order:

@font-face {
    font-family: 'my-web-font';
    src: url('webfont.eot');
    src: url('webfont.eot?#iefix') format('embedded-opentype'),
         url('webfont.woff2') format('woff2'),
         url('webfont.woff') format('woff'),
         url('webfont.ttf') format('truetype'),
         url('webfont.svg#webfont') format('svg');
    font-weight: normal;
    font-style: normal;
}

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For Subversion auto-properties, these can be listed as:

# Font formats
svg   = svn:mime-type=image/svg+xml
ttf   = svn:mime-type=application/x-font-ttf
otf   = svn:mime-type=application/x-font-opentype
woff  = svn:mime-type=application/font-woff
woff2 = svn:mime-type=application/font-woff2
eot   = svn:mime-type=application/vnd.ms-fontobject
sfnt  = svn:mime-type=application/font-sfnt