How to Resolve Windows 11 Outlook having Unknown Files?

You may encounter the Windows 11 Outlook unknown file issue if the Windows fail to recognize the file type or the email client fails to open the file correctly.

Sometimes, this appears as a paperclip icon without a recognizable file name or extension, shown as an unknown file.

However, this does not necessarily mean that the file is damaged or corrupted. In this article, we will find out the reasons behind this issue and see how to fix this unknown file issue in Outlook.

Why does Windows 11 Outlook Show the Files as Unknown?

Outlook may show the attachment files as unknown due to several reasons. Some common ones are:

• The file type is not supported or recognized by Outlook or Windows
• The file types (e.g., .exe, .bat, .library-ms) are blocked by Outlook to protect against malware or virus attack.
• Outlook may fail to read the file types correctly due to corruption in the data file (.pst).
• Outdated, damaged, or incorrectly configured Outlook installation.

Ways to Fix Unknown File Attachments in Outlook

Below are the ways that can help you to fix the Unknown File issue in Outlook on Windows.

1. Check if the File Type is Blocked by Outlook

If your email client is showing files that Windows cannot identify, there is no need to delete them immediately. You must first determine from where the file came and whether it is an actual email attachment or part of Outlook’s stored data.  Follow the steps below:

• Open Outlook.
• Open the email containing the attachment.
• Check whether Outlook displays a warning such as “Outlook blocked access to the following potentially unsafe attachments.”
• Note the name of the attachment and file extension (if visible).

Note: If Outlook has blocked the attachment, don’t try to bypass the security restriction. First verify that the email came from a trusted sender.

Once you have identified the sender, you can use a safer alternative to open the file.

2. Save the Attachment and Check its File Extension

If Outlook displays an attachment but does not open it correctly, try saving it to your computer and checking whether Windows can identify the file. Follow the steps below to save the attachments:

• Open the email containing the attachment.
• Right-click the attachment or select its drop-down menu.
• Choose Save As or Download, depending on your Outlook version.
• Select a folder on your Windows 11 computer.
• Save the file.

After saving the file,

• Open File Explorer.
• Navigate to the folder where you’ve saved the attachment.
• Right-click the file and select Properties.
• Check the file name, file type, and file size.

If the file has no recognizable extension, do not randomly add .docx, .pdf, or another extension. You can contact the sender and ask them to confirm the file type or resend the attachment with the correct file name and extension.

3. Update Outlook and Windows 11

Microsoft regularly releases updates to fix application bugs, security issues, and compatibility problems. Sometimes, an outdated version of Outlook or Windows may cause such issues. You can check and install the latest updates for the operating system and the application.

To install Windows updates,

• Open Settings.
• Select Windows Update.
• Check for available updates.
• Install any pending updates.
• Restart the computer if prompted.

To update Outlook,

• Open Outlook or any another Office application, such as Word.
• Go to File > Office Account.
• Select Update Options > Update Now.

After installing available updates, restart Outlook and check whether the unknown file issue has been resolved.

4. Check Faulty Add-in

Add-ins can enhance Outlook functionality by adding extra features. However, sometimes an outdated incompatible, or damaged add-in can interfere with the application and cause issues. You can check if add-ins are the problem by opening Outlook in Safe Mode. To do this press Windows + R to open Run dialog box and type outlook.exe /safe. If your email client starts normally in Safe Mode then you can find and disable faulty add-in. Follow the steps to do so:

• Open Outlook normally, if possible.
• Go to File > Options > Add-ins.
• At the bottom of the window, select COM Add-ins from the Manage drop-down menu and click Go.
• Clear the check box for the installed add-ins to disable them, and click OK.
• Restart Outlook normally.

If Outlook starts successfully after disabling the add-ins, enable them one at a time, restarting Outlook after each one. This can help you identify the add-in that is causing the startup problem. Once you identify the problematic add-in, keep it disabled or remove it.

5. Repair Corrupted PST

Outlook (POP3 account) stores emails, attachments, and other mail items in the PST file. If this file gets corrupted, the application may fail to read the stored data properly, causing the unknown file issue. In this case, you can repair the PST file by using ScanPST or Inbox Repair tool. This comes pre-installed with Outlook and is available in the installation directory.

ScanPST is suitable for fixing minor corruption in the PST file. However, sometimes it may not be able to repair a severely corrupted PST, and in some cases, the repaired file may still have missing or inconsistent mailbox data after the repair. Alternatively, you can consider Outlook PST repair solutions, such as Online PST repair service by Stellar. It can repair one PST file of up to 500 MB for free. The service can recover emails, contacts, calendar entries, attachments, tasks, notes, and other supported mailbox items from a corrupted PST file. You can also preview the recoverable data before downloading the repaired PST. Here’s how to use this service:

• Overall, the process is straightforward. Visit Stellar Online PST Repair Service.
• Click Repair PST File then sign in and upload the corrupted PST file.

Alt-text-pst-repair-online-service-page

• Click Start Repair, the service then analyzes and repairs the file automatically.

Alt-text-repairing

• Once the repair is completed, you can preview the recovered folder structure and mailbox item counts directly in your browser.
• Now you can download the repaired PST file to your computer and open it in Outlook.

Alt-text-repair-complete

The free online service is perfect for a one-time PST repair, though it requires a stable internet connection to upload your files. For data files larger than 5 GB you should use the desktop software, Stellar Repair for Outlook Having. It is designed to repair severely corrupted PST files and recover mailbox items such as emails, attachments, contacts, calendars, tasks, and notes. The software can also recover deleted mailbox items that are still recoverable from the PST file.

The Technician unlock advanced, enterprise-grade features for managing large-scale email data. It allows you to export recovered files directly to Microsoft 365 and Live Exchange Server.

Additionally, this tools to split and compact large PST files, convert OST files to PST, and merge multiple PST files into a single archive. You also get access to advanced data recovery and deep search filtering options.

Conclusion

The Unknown attachment issue in Outlook can be caused by blocked attachments, outdated software, or a corrupted PST file. Start by checking and installing the latest Windows and Outlook updates. If the problem is related to a damaged PST, try Microsoft’s ScanPST tool first. If this tool cannot fix the issue, you can use a more powerful PST repair tool, such as Stellar Repair for Outlook. It can recover emails, attachments, contacts, calendars, and other mailbox data from the corrupted file.

FAQs

1. What does an “Unknown File” error mean in Outlook?

An “Unknown File” error usually means that Outlook cannot properly identify or display an attachment. It can happen because of an unsupported file type, compatibility issues, or a problem with the Outlook data file.

2. Why is Outlook showing a paperclip but no attachment?

This can happen when Outlook has trouble reading or displaying an attachment.

3. What should I do if ScanPST cannot repair my PST file?

If ScanPST fails or the PST is still inaccessible, you can consider a specialized PST repair tool such as Stellar Repair for Outlook. It is designed to recover mailbox data from corrupted PST files.

5. Is it safe to open an unknown Outlook attachment?

You should be careful, especially if the email comes from an unknown or unexpected sender. Do not open suspicious executable or script files until you have verified the sender and the file.