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CantOpen

What is this file?

Drop a file you can’t open. We look at its contents, not its name, tell you what it is and which program made it, and how to open it. If it is an Outlook mailbox or message (.pst, .ost, .olm, .msg), an MBOX mailbox, an .eml email, a winmail.dat, a medical image or a Publisher file, you can open it right here, and convert a mailbox to PST.

Drop a file here

or

Your file stays on this device: it is read here, in your browser, and never uploaded.

We read only the start of the file (and, for ZIP and Office files, the list of files inside), in your browser, to tell what it is. Up to 20 files at once.

How it works

  1. Drop the file

    Or several. They stay on your device: the check runs in this browser tab and nothing is uploaded.

  2. We read its signature

    Most formats start with a few typical bytes, or have a typical structure inside. We match those against the formats on this page.

  3. You get an answer

    What the file is, who usually sends it, and the simplest way to open it, with a link to our viewer when we have one.

Formats we recognize

The ones people most often get stuck on are first. Those with a link open directly on this site.

Email and Outlook

Medical images

Office and documents

  • Microsoft Publisher (.pub)

    Newsletters, flyers and brochures from a program Microsoft retired in October 2026.

  • Primavera P6 schedule (.xer)

    Project schedules exported from Oracle Primavera P6: activities, WBS and a Gantt chart.

  • Markdown (.md, .markdown)

    Notes, READMEs and the files ChatGPT, Claude and Cursor write: shown formatted, saved as PDF or Word.

  • ChatGPT or Claude data export (.zip)

    Your chat history: search it, read any chat, save chats as PDF, Word or Markdown.

  • Microsoft Project (.mpp)

    Project schedules.

  • XPS and OpenXPS (.xps, .oxps)

    Windows’ “print to XPS” documents.

  • Word, Excel, PowerPoint, Visio

    Old (97–2003) and current formats, and password-protected files.

  • OpenDocument (.odt, .ods, .odp)

    LibreOffice and OpenOffice files.

  • Signed files (.p7m, .p7s)

    Documents in a digital signature envelope.

  • PDF, RTF, WordPerfect, EPUB

    And other everyday document formats.

Maps and data

Pictures, archives and the rest

  • HEIC, WebP, AVIF

    iPhone photos and web images.

  • ZIP, RAR, 7z, ISO, DMG

    Archives and disc images.

  • Audio and video

    MP4, MOV, MKV, MP3, WAV, Ogg and more.

  • Programs and shortcuts

    So you know not to run them.

  • Anything else

    We show the first bytes and any readable text, which often name the program.

Why a file won’t open

The name says one thing, the contents another

Your computer decides which program opens a file from the end of its name (the extension). If the extension is missing, wrong, or one your computer does not know, nothing happens or the wrong program complains. The contents tell the truth: almost every format starts with a few bytes that identify it. A PDF starts with “%PDF”, a ZIP with “PK”, an Outlook data file with “!BDN”.

That is what this page reads. We look at the first 64 KB of the file (and, for ZIP and Office files, the list of files inside) and compare them with the formats we know. When the name and the contents disagree, we say so.

The program that made it is not on your computer

Some formats belong to one program on one system. An Outlook .pst or .ost needs Outlook for Windows, a Publisher .pub needs Publisher, a Project .mpp needs Project. A Mac, a Chromebook, a phone or a PC without the right Office edition cannot open them, and these programs are expensive to buy for one file.

Files that are not really documents

Some attachments are wrappers. A winmail.dat holds the attachments an Outlook user meant to send. A .p7m holds a signed document. A medical CD holds a DICOMDIR index and hundreds of image files without names that make sense. The file you need is inside, and the right tool unpacks it.

Damaged, incomplete or encrypted

An interrupted download leaves a file that starts correctly and ends too early; an empty file (0 bytes) is a failed copy. Password-protected Office files and encrypted emails are encrypted: no viewer can open them without the password or key. In those cases the fix is to get the file again, or the password, from the sender.

When not to open a file

If the file turns out to be a program (.exe, .msi, .apk), a shortcut (.lnk) or a web page you did not expect, do not run it: that is how malware spreads. This check is safe because it only reads bytes; it never runs the file.

Frequently asked questions

Is my file uploaded?

No. The check runs in your browser; the file never leaves your device. No code on the page sends the file or its name anywhere, not even to our own domain, and the page’s security rules let it connect only to our domain and to two analytics services you can refuse. The analytics, the cookie-free visit statistics and the error report sent if the page breaks record only page-level or technical data; the analytics also receive the tool events described in the privacy policy (which step was used, the kind of file, whether it worked; if you are signed in, which account), never your file, its name, its size or its contents.

How can you tell what a file is without opening it?

Formats have signatures: fixed bytes at the start, or a typical internal structure. We read the first 64 KB (for ZIP and Office files, also the list of files inside) and compare. We never run the file.

What if you can’t recognize my file?

We show the first bytes and any readable text in them. Program names, version numbers or words like “Microsoft” often appear there and tell you which program to ask about.

My file ends in .dat. What is it?

“.dat” is a generic name for data. If it came attached to an email, it is very often a winmail.dat from an Outlook user, which we can unpack. Drop it above to check.

Can you open the file for me?

For Outlook .pst, .ost and .msg files, Outlook for Mac .olm archives, MBOX mailboxes, .eml messages, winmail.dat, DICOM images and Microsoft Publisher .pub files: yes, in this browser, on the pages linked above, and mailboxes can be converted to an Outlook .pst. For the other formats we tell you which free or common program opens them.

Does it cost anything?

Identifying and viewing files, and saving single messages, attachments and pages, are free and need no account. Work on many items at once is paid: bulk export of a mailbox, converting many emails or Publisher files, the PDF of a Publisher file over two pages, every page of a Publisher file as PNG in one ZIP, the conversion to PST, the exports of a Primavera schedule, converting a folder, ZIP or vault of Markdown files at once, and exporting many chats of a ChatGPT or Claude export at once. A single export costs $5.99: you pay once for one export (one file, mailbox or set of files, with every output it makes) and can save it again later at no extra cost. An unlimited plan costs $9.99 a month or $49 a year and renews until you cancel it (if the price ever changes, we tell you before the renewal it applies to); you can cancel any time in your account. Prices include VAT or GST where it applies.

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