Outlook attachments not opening: how to fix it (classic, new and web)

Applies to: classic Outlook for Microsoft 365, Outlook 2024, 2021 and 2019 on Windows 11 and Windows 10, the new Outlook for Windows, and Outlook on the web. Where a step only works in one of them, the section says so.

Microsoft documents three of the causes covered here: blocked file types, the secure temporary folder and winmail.dat. The other steps are standard Windows and Office troubleshooting, and we say where a step is our suggestion rather than Microsoft guidance. If Outlook itself won’t start, our guide to Outlook not opening at all is the better place to begin.

Match your symptom

What you see	Likely cause	Go to
“Outlook blocked access to the following potentially unsafe attachments” or a paperclip with no file	The file type is on Outlook’s block list	Fix 1
Word, Excel or PowerPoint opens the file but you can’t edit it, with a yellow or red bar at the top	Protected View or a blocked macro	Fix 2
Windows asks “How do you want to open this file?” or nothing happens on double-click	No default app for that file type	Fix 3
The reading pane says the file can’t be previewed, but it opens fine	The previewer for that type is missing, off or blocked	Fix 4
“Cannot create file” when you open or save an attachment in classic Outlook	The secure temporary folder is full or locked	Fix 5
Attachments fail in Outlook but open after saving them to the desktop	An add-in, security software or damaged Office files	Fix 6, Fix 7
The only attachment is winmail.dat or ATT00001.dat	The sender used Rich Text format (TNEF)	Fix 8
Nothing opens in the new Outlook or on the web	App data, browser or account-side problem	Fix 7, Outlook on the web

Fix 1: The file type is blocked

Outlook refuses to open some kinds of files no matter who sent them. Microsoft’s page Blocked attachments in Outlook explains that attachments are a common way for viruses to spread, so Outlook blocks file types that can run code or change your system. The list on that page has well over a hundred extensions. Among them are:

• Programs and scripts: .exe, .bat, .cmd, .com, .scr, .js, .vbs, .wsf, .hta, .ps1 and Python files such as .py.
• Installers and system files: .msi, .msp, .msu, .cpl, .inf, .reg, .lnk and .url.
• Disk images: .iso, .img, .vhd and .vhdx.
• Other types: several Access database extensions, .jar, .apk, .appx, .chm and .pst.

This is a safety feature, not a fault, so the fix is to get the file to you in a different form. Microsoft’s page suggests these options; we’ve listed them from safest to least safe:

1. Share a link instead. The sender uploads the file to OneDrive or SharePoint and sends a link rather than an attachment. Nothing is blocked because no file travels with the email.
2. Send a compressed archive. The sender right-clicks the file and chooses Send to > Compressed (zipped) folder in Windows, then attaches the .zip. You save the archive and extract it. Some mail servers also inspect .zip contents, so this doesn’t always get through.
3. Ask your administrator. With a work or school (Exchange) account, Microsoft says the mail administrator can allow certain file types for your mailbox. That keeps the decision with someone who can judge the risk.
4. Rename the extension, with care. Microsoft also describes renaming the file to a harmless extension before sending and renaming it back after saving. Only do this when you know the sender and expected that exact file, because it hides the very thing the block is meant to warn you about.

Security warning about registry changes: Microsoft’s page mentions an advanced registry procedure for non-Exchange accounts that unblocks file types (it points to Microsoft Support article 829982). We don’t give those steps here. Unblocking a type such as .exe or .js lets every future message with that kind of file open with a double-click, including malicious ones. If you really need it, read Microsoft’s article, back up the registry first and unblock only the single extension you need, never a whole group.

New Outlook and Outlook on the web: Microsoft’s blocked-attachments page lists Outlook on the web but doesn’t describe a separate block list for it or for the new Outlook for Windows. If a file type won’t open there, the same zip, OneDrive and administrator options above are the practical routes.

Fix 2: The file opens read-only in Protected View

Office documents that arrive as attachments usually open in Protected View: a read-only mode with a yellow message bar at the top. It isn’t an error. Office treats files from email as potentially unsafe and keeps editing, macros and some content switched off until you choose otherwise.

1. Check that you expected the file and know the sender.
2. Select Enable Editing in the yellow bar. The document becomes a normal, editable file.
3. If the bar is red, or says the file can’t be opened in Protected View, save the attachment to your PC first and open it from File Explorer. A red bar can also mean the file failed a validation check, in which case it’s safer to ask the sender for a new copy.

Avoid switching Protected View off in the Trust Center just to save a click. It applies to every file from the internet, not only the one you’re working with. If the bar mentions blocked macros rather than Protected View, see our guide to macros blocked from an untrusted source.

Fix 3: No app is set to open the file type

Outlook doesn’t open attachments itself; it hands them to whatever app Windows has linked to that extension. If no app is linked, Windows shows How do you want to open this file? or Outlook does nothing visible. This is common with .heic photos, .dwg drawings, .eml messages or older formats.

1. In classic Outlook, right-click the attachment and choose Save As. In the new Outlook or on the web, use the attachment’s menu and choose Download.
2. Right-click the saved file in File Explorer and choose Open with > Choose another app.
3. Pick an app that handles the type and select Always. If none is listed, install a suitable viewer from the vendor or the Microsoft Store first.
4. Go back to Outlook and open the attachment again.

To manage this for several file types at once, our guide on how to change default apps in Windows 11 covers the Settings > Apps > Default apps page. If a PDF or image opens in the wrong program rather than none, the same page fixes that.

Fix 4: The attachment won’t preview

Classic Outlook can show many attachments inside the reading pane through small add-ons called previewers. When a preview fails with a message such as “This file cannot be previewed”, the file is usually fine and opens normally with a double-click. The causes are a missing previewer for that type, a previewer turned off, or a preview handler installed by another program that no longer works.

1. In classic Outlook, go to File > Options > Trust Center > Trust Center Settings.
2. Select Attachment Handling.
3. Make sure Turn off Attachment Preview is cleared.
4. Select Attachment and Document Previewers and check that the previewer for the file type is selected. Clear any third-party previewer you no longer use.
5. Select OK, close Outlook and open it again.

PDF previews depend on the PDF app you install, because that app supplies the previewer. If PDF previews stopped after you changed or removed a PDF reader, reinstalling or updating it is the likely fix; we suggest this based on how previewers are registered, not on a Microsoft article. Windows can also refuse to preview files downloaded from the internet, a separate change covered in our guide to “The file you are attempting to preview could harm your computer”.

Fix 5: Clear the secure temporary folder (classic Outlook)

When you open an attachment in classic Outlook, it saves a temporary copy to a hidden folder and opens that copy. Microsoft calls it the Outlook Secure Temporary File folder. Its archived article Attachment issues in Outlook describes two problems with it: copies can stay in the folder after Outlook closes, and opening or saving an attachment can fail with Cannot create file followed by a request to check your folder permissions.

Version note: that Microsoft article officially applies to Outlook 2003, 2007 and 2010 and says the fixes shipped in updates for those versions. We haven’t found a current Microsoft article for Outlook in Microsoft 365, so treat this step as worth trying rather than a documented fix for newer versions.

1. Close every open attachment, then exit Outlook.
2. Press Windows key + R, type regedit and select OK. Read-only use is enough here; don’t change anything.
3. Go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Security. The article uses 14.0 for Outlook 2010; 16.0 is the version number used by current Office, which is our assumption for newer releases.
4. Find the value OutlookSecureTempFolder and copy the folder path it contains.
5. Paste the path into the File Explorer address bar and press Enter. The folder is hidden, so typing the path is easier than browsing.
6. Delete the files inside the folder. Keep the folder itself.
7. Start Outlook and try the attachment again.

If the value isn’t there, Microsoft’s article says Outlook uses a randomly named subfolder of a Content.Outlook folder under the Temporary Internet Files location. On current Windows versions that location has moved, so the registry value is the more reliable pointer. Files in this folder are copies, but if you edited an attachment and never saved it elsewhere, your changes may only exist there, so check before you delete.

Fix 6: Test safe mode and your antivirus

If attachments open after you save them to the desktop but fail from inside classic Outlook, something loaded into Outlook may be interfering. Safe mode starts Outlook without add-ins, which makes it a quick test.

1. Exit Outlook.
2. Press Windows key + R, type outlook.exe /safe and select OK.
3. Choose your profile if asked, then open the attachment.

If it opens in safe mode, go to File > Options > Add-ins, choose COM Add-ins next to Manage and select Go. Clear the add-ins, restart Outlook normally and turn them back on one at a time until the problem returns. Our Outlook won’t open guide walks through the same add-in test in more detail.

Security software often adds an email-scanning add-in to Outlook or scans attachments as they’re saved to the temporary folder. If a security add-in shows up in the list, update the product and contact its vendor if the problem continues. Don’t turn off antivirus protection to open an attachment you’re unsure about; that removes the one check that could stop a harmful file.

Fix 7: Update and repair Office or the new Outlook

When attachments fail for every file type and safe mode doesn’t help, damaged Office files are a possible cause.

• Classic Outlook: install updates through File > Office Account > Update Options > Update Now, then run a repair from Settings > Apps > Installed apps: find Microsoft 365 or Office, choose Modify and start with Quick Repair, then Online Repair if needed. Our step-by-step guide to repairing Microsoft Office explains the difference.
• New Outlook for Windows: in Settings > Apps > Installed apps, find Outlook (new), open Advanced options and select Repair. Use Reset only if Repair doesn’t help, since it clears the app’s local data and you sign in again afterward.
• Check the attachment itself. If one file fails everywhere, including on another PC, it may be damaged or incomplete. Ask the sender to send it again or share it as a link.

Fix 8: The attachment is called winmail.dat

A winmail.dat attachment, sometimes shown as ATT00001.dat or similar, means the message was sent in Rich Text format from Outlook. Microsoft’s article How message format affects email messages explains that Rich Text messages use a Microsoft format called TNEF, which packs formatting and attachments into one binary file. Mail apps that can’t read TNEF show that file as winmail.dat, and the real attachments are hidden inside it.

The lasting fix is on the sender’s side. Ask them to do one of the following and send the message again:

• In classic Outlook, go to File > Options > Mail and set Compose messages in this format to HTML or Plain Text.
• For a single message, on the Format Text tab, choose HTML or Plain Text before sending.
• If the sender is on Exchange or Microsoft 365, their administrator can also control how TNEF is sent to outside recipients.

Microsoft notes that meeting requests, voting buttons and task requests still need TNEF, so a winmail.dat can still appear on those. The article also points out that winmail.dat can include the sender’s .pst file path and sign-in name, which is one more reason for senders to switch to HTML. Third-party winmail.dat viewers exist, but we haven’t reviewed any, and asking for a resend is simpler and safer.

Outlook on the web as a test

Signing in to Outlook on the web in a browser is a quick way to narrow the cause; this is our suggestion. If the attachment opens or downloads there, the mailbox and file are fine and the problem is on your PC (Fixes 3 to 7). If it fails on the web too, the file may be blocked by the server or damaged, which points back to Fix 1 or a resend. On the web, try another browser or a private window, since browser extensions and cached data can stop downloads or previews.

If attachments stopped opening right after an Office update, Microsoft’s page of fixes and workarounds for recent issues in classic and new Outlook is worth checking. This guide doesn’t list a specific known issue, because none had been confirmed on an official page for it as of November 2026.

Related guides

• How to change default apps in Windows 11 (browser, PDF, photos)
• How to repair Microsoft Office (Quick Repair and Online Repair)
• Outlook won’t open: how to fix it in Windows 11
• “The file you are attempting to preview could harm your computer”: fix
• “Microsoft has blocked macros from running because the source of this file is untrusted”: fix

Official sources

• Microsoft: Blocked attachments in Outlook
• Microsoft Learn: Attachment issues in Outlook (Secure Temporary File folder, archived)
• Microsoft Learn: How message format affects email messages (TNEF and winmail.dat)