OpenType Font MIME Type

What does font/otf mean?

The font/otf media type identifies an OpenType font with OpenType Layout capabilities. OpenType is a table-based, scalable font format. A file can contain character-to-glyph mappings, names, metrics, vector outlines, layout rules, variation axes, color glyph data, and other information used to shape and render text.¹²

The usual extension is .otf. The IANA registration also allows .ttf because both extensions can carry OpenType data. In normal file naming, .ttf usually indicates TrueType outlines, while .otf can contain TrueType, Compact Font Format (CFF or CFF2), or SVG-supported outlines.¹

How to identify an OpenType font

OpenType uses the sfnt container inherited from TrueType. The first four bytes are the sfntVersion field, followed by a directory of tagged tables.²

Neither signature is unique to the .otf suffix. A correctly named .otf may begin with either value, while a .ttf OpenType font normally begins with 00 01 00 00. Collections use .otc or .ttc and the separate font/collection media type.¹²

The table directory is more informative than the filename. Common required tables include cmap for mapping characters to glyphs, name for human-readable names, head for the font header, and hmtx for horizontal metrics. TrueType outlines normally use glyf and loca; CFF-based fonts use CFF or CFF2.²

Where OpenType came from

Microsoft and Adobe developed OpenType as an extension of TrueType that could also contain PostScript font data. The first OpenType specification was published in April 1997.³⁴ The format later became the basis of ISO/IEC 14496-22, the Open Font Format standard referenced by the font/otf registration.¹

OpenType's significance is not just that it combines outline technologies. Its layout tables can describe glyph substitution and positioning needed for ligatures, marks, alternate forms, kerning, and complex writing systems. Modern OpenType can also represent variable fonts, color glyphs, mathematical typography, and large Unicode repertoires.⁴

The MIME type came later. RFC 8081 created the top-level font media type and registered font/otf in February 2017. Before that registration, servers and software used a mixture of generic or unofficial labels. New deployments should send font/otf, not labels such as application/x-font-opentype.¹⁵

Older configurations may also use application/x-font-otf, application/x-font-opentype, or application/vnd.ms-opentype. These are not registered aliases for font/otf; RFC 8081 does not list them as deprecated registrations. The registered application/font-sfnt is a separate, deprecated generic font type whose replacement is font/sfnt, not font/otf.⁵⁶ Use font/otf for an OpenType font.

How to open, install, or inspect an .otf file

On macOS, Font Book can install, validate, preview, and show the character repertoire of OpenType and OpenType-SVG .otf files.⁷ On Windows 10 and 11, the Fonts settings and the file's Install command support OpenType .otf fonts.⁸ Design applications such as Adobe Illustrator can expose OpenType features including ligatures and alternates, although the available controls depend on what the individual font contains.⁹

For a quick inspection, use a font manager or validator rather than changing the extension. Check at least:

• The family, style, version, designer, and license entries in the name table.
• The character repertoire and missing glyphs.
• The outline flavor and whether variable-font axes are present.
• OpenType Layout features for every script and language you need.
• Validation warnings before installing or distributing the font.

Current Chrome, Edge, Firefox, Opera, and Safari can use OpenType resources through CSS @font-face. Serve the file with Content-Type: font/otf, and configure cross-origin access when the font is hosted on another origin. For production web delivery, WOFF2 is usually the better choice because it compresses font data more efficiently.¹⁰

@font-face {
  font-family: "Example Family";
  src: url("/fonts/example.otf") format("opentype");
  font-display: swap;
}

Support in Poly

Poly displays a rendered specimen for an .otf, with details such as its family, format, weight, style, glyph count, and estimated language coverage. You can find fonts by filename, filter for font files, and search for visually similar specimens. The agent can view the specimen and its caption, which helps answer questions about the typeface's visible character. Poly also recognizes older labels including application/x-font-otf, application/x-font-opentype, and application/vnd.ms-opentype as OpenType fonts.

The details shown are not a full font inspection. Poly checks one face, so a multi-face font collection preview describes its first face. Variable fonts are represented at their default variation settings. Font metadata is available in the file details panel but is not searchable as text; the preview also cannot expose every glyph, feature, license field, or font table. Open or download the file in a font application when those details matter.

OTF, TTF, WOFF, and WOFF2 compared

These labels overlap, so the extension alone is an imperfect technical description.

OpenType is the underlying font model in all four cases more often than the names suggest. WOFF and WOFF2 package sfnt font data for web delivery rather than defining an unrelated set of glyphs. Converting a valid .otf to WOFF2 can reduce transfer size without redesigning the typeface, but the conversion tool must preserve the tables and features your site needs.¹⁰

Security, privacy, and licensing

A font is not merely a passive picture. RFC 8081 notes that TrueType hinting instructions can act as executable code inside a font interpreter, and malformed table structures can consume excessive CPU or memory. Treat fonts from unknown sources as untrusted input, keep the operating system and font software updated, and validate a file before installing it.⁵

Font files can also contain arbitrary or identifying metadata. The format itself provides no confidentiality, so do not assume that subsetting or renaming a font removed private information.⁵

Conversion does not grant permission to publish. A desktop license may allow local installation while prohibiting web embedding, redistribution, modification, or format conversion. Read the font's license and embedding permissions before uploading it to a public server or sharing a converted copy.