Files that will not open, by format
Someone sent you a file and double-clicking it does nothing. What you need at that point is not the history of the format but two answers: why it will not open, and how to see what is inside. The formats here are the ones people actually get stuck on, and each page is built around those two questions.
If the file has no extension, or one you do not recognise, start with the file identifier. It reads the first bytes of the file to work out the real format and points you to the matching page when there is one. Identification runs in your browser; the file is not sent anywhere.
Email attachments
- winmail.dat
How to Open a winmail.dat File
winmail.dat is not a document. It is an entire email — body, formatting and every attachment — packed into one file by Microsoft Outlook, and only Outlook unpacks it automatically.
- .dat
How to Open a .dat File
.dat is not a format. It is a placeholder extension that any program can use for its own data, so two .dat files may have nothing in common. The first step is not opening it but identifying it.
- .msg
How to Open a .msg File Without Outlook
A .msg is one Outlook message saved as a file. It is not a standard email format: inside it is a compound file, the same container Microsoft used for .doc and .xls, which is why nothing else opens it by default.
- .mime.mht.mhtml
How to Open a .mime File
A .mime file is an email in the format email actually travels in: headers, then the body, then any attachments, all encoded as text. It is the same thing as a .eml, just wearing a different name.
- .p7m.p7s
How to Open a .p7m File
A .p7m is not a document — it is an envelope. Inside is a real file (usually a PDF), wrapped in a digital signature, encryption, or both. What you can do with it depends entirely on which.
Apple iWork documents
- .pages
How to Open a .pages File on Windows
A .pages file is a document written in Apple Pages. It is not a Word file and Word cannot read it, but you do not need a Mac to get at the contents.
- .numbers
How to Open a .numbers File in Excel
A .numbers file is a spreadsheet from Apple Numbers. Excel cannot open it, and converting is less clean than with a word processor document because Numbers is structured differently from Excel in a way that has no direct translation.
- .key
How to Open a .key File (Keynote) on Windows
A .key file is a Keynote presentation. PowerPoint cannot open it, and converting costs you exactly the thing Keynote is usually chosen for: the transitions.
Other files from Apple devices
- .aae
What Is an .aae File on iPhone?
An .aae file is a list of the edits you made to a photo — crop, filter, exposure — stored separately from the image itself. It is not a picture, and on its own it shows nothing.
- .webarchive
How to Open a .webarchive File on Windows
A .webarchive is a whole web page — HTML, images, stylesheets — saved into a single file by Safari. Only Safari opens it directly, which is a problem the moment the file reaches Windows or Android.
- .DS_Store
What Is .DS_Store, Can You Delete It, and What Does It Reveal
A .DS_Store file holds the Mac Finder's view settings for one folder — icon positions, view mode, window size. It is created automatically, it is safe to delete, and it lists every file Finder saw in that folder, which is why it matters when it leaves the Mac.
Documents from Japanese office software
- .xdw.xbd.xct
How to Open an .xdw File (DocuWorks)
An .xdw is a document from DocuWorks, the document-handling software from FUJIFILM Business Innovation (formerly Fuji Xerox). You do not need DocuWorks itself: the free DocuWorks Viewer Light shows and prints it.
- .jtd.jtdc.jtt.jttc.jtsd
How to Open a .jtd File (Ichitaro)
A .jtd is a document from Ichitaro (一太郎), the word processor from JustSystems. You do not need Ichitaro itself: the free Ichitaro Viewer for Windows shows and prints it.
Design and CAD
- .psd.psb
How to Open a PSD File Without Photoshop
A .psd is a Photoshop document: not a picture but a stack of layers, masks and adjustments. That structure is why ordinary image viewers refuse it, and why it can be enormous.
- .clip
How to Open a .clip File
A .clip file is a layered Clip Studio Paint document. Only Clip Studio Paint opens it properly — but the format is less opaque than it looks, and there is usually an easier way to get what you need.
- .dwg
How to Open a DWG File Without AutoCAD
A .dwg is a CAD drawing in AutoCAD's native format. It is not an image — it holds geometry, layers and dimensions as data — and Autodesk provides free viewers even though AutoCAD itself is expensive.
Windows files and archives
- .lnk.ink
How to Open a .lnk File (and Why .ink Does Not Exist)
A .lnk file contains no document. It is a Windows shortcut: a small record of where some other file lives. If you received one on its own, you received the signpost without the destination.
- .bak
How to Open a .bak File
.bak is not a format. It is a suffix that any program can add to a copy it is keeping, so what is inside depends entirely on what made it. The first step is identifying it, not opening it.
- .rar
How to Open a RAR File
A .rar is a compressed archive holding one or more files. Because the format belongs to one vendor, Windows 10 and macOS cannot open it out of the box the way they open a ZIP; Windows 11 extracts it after its late-2023 update.
- .lzh.lha
How to Open an LZH File on Windows 11 and Mac
An .lzh (or .lha) file is an archive made with LHA, the compression tool that was standard in Japan through the 1990s and 2000s. Current Windows and macOS cannot open it, but it is easy to extract, and it is safe as long as you do not run what comes out of it.
- .xps.oxps
How to Open .xps and .oxps Files (Windows 11 Has No Viewer by Default)
An .xps file is a finished document laid out for print, Microsoft's answer to PDF from the Windows Vista era. .oxps is the same idea in the later standardised form. Windows still understands both, but since 2018 the viewer has to be installed by hand.
- .crdownload.part.download
What Is a .crdownload File, and Can You Open It?
A .crdownload file is a download that Chrome or Edge has not finished — either it is still going, or it was cut off. Until the last byte arrives the browser keeps the file under this name so nothing opens it half-written.
- Thumbs.db
What Is Thumbs.db, Can You Delete It, and What Is Still Inside
Thumbs.db is a cache of small preview images that Windows Explorer builds when a folder is shown in thumbnail view. It is not one of your files, it is safe to delete, and it holds thumbnails of pictures that may no longer exist.