Mobipocket Ebook MIME Type

What does application/x-mobipocket-ebook mean?

The application/x-mobipocket-ebook media type identifies a Mobipocket ebook, usually stored in a .mobi file. Mobipocket introduced the proprietary binary format in 2000 for distributing reflowable books to handheld devices. It packages book text derived from HTML, images, navigation, and publication metadata in a container based on Palm database conventions.¹

The name is widely recognized but is not an IANA registration. Its x- prefix marks it as a historical, nonstandard identifier. IANA instead registers application/vnd.amazon.mobi8-ebook for the later Kindle Format 8 format commonly carried in .azw3 files.²

How to identify a MOBI file

The usual extension is .mobi. Older Mobipocket books may use .prc, but PRC is a generic Palm container extension and does not prove that a file is an ebook. calibre, for example, only treats a PRC as an ebook when it contains a TEXTREAD or MOBIBook header.³

For classic Mobipocket files, the most useful signature is the ASCII string BOOKMOBI, represented in hexadecimal as 42 4F 4F 4B 4D 4F 42 49, beginning at byte offset 60. Some older files instead carry TEXTREAD at the same location. The filename and signature should be considered together because .prc is ambiguous.¹

Renaming a different ebook to .mobi does not convert it. It only changes the filename.

Where MOBI came from

Mobipocket SA created the format for early Palm and Windows CE readers. The company compiled Open eBook source material into a compact binary package, initially called XDOC PRC. Amazon acquired Mobipocket in 2005 and extended the technology for Kindle delivery.¹

That history produced several terms that are easy to confuse:

MOBI remained useful because text could reflow for small screens and older readers could ignore newer sections in a combined file. The tradeoff is a format whose binary details were never fully published and whose internal layout changed across generations.¹

Amazon has now retired MOBI as a publishing input. It ended support for reflowable MOBI submissions in 2021 and stopped accepting fixed-layout MOBI files on March 18, 2025. Amazon recommends EPUB, DOCX, or KPF for new Kindle publications.⁴

How to open an application/x-mobipocket-ebook file

Web browsers do not have a native MOBI reading mode, so use an ebook application rather than expecting the file to render in a tab.

• calibre can view MOBI and convert MOBI or qualifying PRC books to EPUB, PDF, DOCX, and other formats.³
• FBReader opens .mobi files on desktop, Android, and iOS, but only when they are not protected by Mobipocket DRM.⁵
• An older Kindle or Mobipocket-compatible reader may open an unprotected file, although support varies by device generation and by which MOBI sections the file contains.

Support in Poly

Poly recognizes application/x-mobipocket-ebook directly and maps .mobi filenames to that type. It classifies MOBI as a document and converts it for the rest of Poly's document features.

After a successful conversion, Poly:

• Renders pages for thumbnails and the in-app PDF viewer.
• Extracts page text, with OCR fallback where needed.
• Indexes text and page-level visual similarity.
• Makes the converted pages and text available to the Poly agent.

The preview is not a native reflowable ebook reader. Font resizing, reader themes, and other MOBI-specific behavior are therefore not preserved as interactive controls.

Poly's filename inference recognizes .mobi, but not .prc. A .prc file is also inherently ambiguous, so upload it with an accurate MIME type or rename it only after confirming that its internal signature identifies a Mobipocket book. DRM-protected files and malformed files cannot be converted, which limits them to ordinary file storage and filename search.

Converting MOBI to EPUB or PDF

For an editable, reflowable successor, convert an unprotected MOBI book to EPUB. For a fixed-layout copy that is easy to share or print, choose PDF. calibre's command-line converter uses the output extension to select the result format:⁷

ebook-convert book.mobi book.epub
ebook-convert book.mobi book.pdf

Conversion is not always lossless. MOBI has limited table-of-contents semantics, and combined files may contain more than one representation of the book. Compare chapter order, navigation, images, footnotes, and accented characters after conversion. calibre documents that classic MOBI commonly simulates a metadata table of contents with an additional inline table near the end of the book.⁸

Is .prc the same as .mobi?

Not always. Early Mobipocket ebooks often used .prc, and compatible tools may open those files as MOBI books. But Palm software also used PRC containers for applications and other data. Check for BOOKMOBI or TEXTREAD at offset 60 instead of deciding from the extension alone.¹³

Is MOBI still a good format for new ebooks?

Usually not. Existing unprotected MOBI files remain readable in applications such as calibre and FBReader, so the format is still in use as legacy content. For a new publication or a preservation copy, EPUB offers a documented, actively maintained path and is accepted by current Kindle publishing workflows. Keep MOBI when an older device specifically requires it; otherwise, migrate a verified copy and retain the original.