Kindle Format 8 MIME Type

What does application/vnd.amazon.mobi8-ebook mean?

The application/vnd.amazon.mobi8-ebook media type identifies an Amazon Kindle Format 8 ebook. The usual extension is .azw3, so the format is also commonly called AZW3 or KF8. Amazon created KF8 as a proprietary Kindle format with substantially richer HTML and CSS support than classic Mobipocket books.¹²

IANA registered the media type on July 1, 2016 for packaging ebooks. The registration names .azw3 as its extension, lists no magic number, and records application/x-mobi8-ebook as a deprecated alias.¹

Where Kindle Format 8 came from

Amazon announced Kindle Format 8 in September 2011 and introduced it the following month. The Book Industry Study Group described it as a proprietary, HTML5- and CSS3-based format that resembles EPUB 3 in some respects but is not EPUB 3.³

That newer presentation model brought features that old MOBI could not express as well. Amazon's current publishing documentation distinguishes amzn-kf8 from amzn-mobi in CSS media queries, allowing a combined source to provide complex KF8 styling and simpler fallback rules for older MOBI readers.⁴ Amazon also documents broad KF8 support for semantic HTML elements, embedded SVG, fonts, positioning, opacity, borders, and other CSS features. It does not support every web feature: scripts are reserved for Amazon use, while audio, video, and canvas elements are not supported.⁵

How to identify and open an AZW3 file

The best portable identifier is the .azw3 extension paired with the registered media type. IANA explicitly says the format has no magic number, so a short signature check cannot authenticate it on its own.¹ A file may also be encrypted or protected with Kindle DRM even when its extension is correct.

Web browsers do not include a native AZW3 reader. Use software that understands Kindle ebooks instead:

• Amazon Kindle applications and compatible Kindle devices are the format's intended readers.¹
• calibre can open unprotected AZW3 books and convert them to formats such as EPUB, PDF, or DOCX. Its conversion documentation lists AZW3 as both an input and an output format.⁶

Support in Poly

Poly recognizes both the canonical application/vnd.amazon.mobi8-ebook identifier and .azw3 filenames. It classifies the file as an ebook, then calls calibre's ebook-convert to create a PDF derivative. That derivative drives Poly's document pipeline.

After conversion succeeds, Poly:

• Generates a first-page thumbnail and rendered page images.
• Opens the converted copy in the in-app PDF viewer.
• Extracts the PDF text and applies OCR fallback to rendered pages.
• Indexes text and page-level visual similarity.
• Lets the Poly agent read extracted text alongside rendered page images.

The conversion changes the reading experience. Poly shows fixed PDF pages rather than a native Kindle view, so adjustable fonts, themes, reflow, Kindle navigation behavior, and format-specific layout controls are not interactive. Files larger than 1 GB are not converted, and document processing covers at most 1,024 pages. DRM-protected, malformed, or otherwise unconvertible files remain stored and searchable by filename, but their book contents are not previewed, indexed, or available to the agent.

Converting AZW3 to EPUB or PDF

For an unprotected book, EPUB is usually the better editable and portable destination. PDF is useful when you need fixed pages for review, printing, or sharing. calibre chooses the output format from the destination extension:⁶⁸

ebook-convert book.azw3 book.epub
ebook-convert book.azw3 book.pdf

Conversion is not a guarantee of identical rendering. KF8 allows Amazon-specific metadata and CSS behavior, while EPUB uses a different standardized package model. PDF also freezes a reflowable book into pages. Check the table of contents, chapter order, fonts, images, footnotes, page breaks, and right-to-left or vertical text after conversion.

Is AZW3 the same as MOBI?

No. calibre notes that MOBI books can contain either older Mobi6 content or KF8 content, and a combined .mobi can carry compatibility representations. The .azw3 extension, however, conventionally identifies the newer Kindle Format 8 form and has its own registered media type.¹⁶

The distinction matters when serving downloads or importing files. Labeling an old MOBI book as application/vnd.amazon.mobi8-ebook does not add KF8 features, and renaming it to .azw3 does not convert its contents.

Is AZW3 a good archival format?

AZW3 remains useful when a Kindle device or an existing library requires it. It is less attractive as a preservation master because Amazon has not published the format specification, and IANA explicitly records interoperability as dependent on Amazon's implementation choices.¹ For long-term access, retain the original and make a verified EPUB or PDF derivative when copyright and DRM permit. EPUB is openly documented and broadly implemented, while PDF preserves a particular paginated appearance.