Email Attachments Won’t Open After Upgrading to Windows 10 (Fixed)

This specific issue was fixed in the August 19 2015 Product Update.

After upgrading to Windows 10, some users are having problems opening Office documents that are attached to Outlook messages. When they try to open the attachments, they receive an error message that there is an error or the file is corrupt.

Excel: "This file is corrupt and cannot be opened"
Word: "Word experienced an error trying to open the file"
PowerPoint: "The application was unable to start correctly", "PowerPoint found a problem with <filename>", "Sorry, PowerPoint can’t read <filename>"

The files are not corrupted but there are issues with the file permissions after the upgrade that affect opening the files in Protected View. Protected View is enabled by default when opening file attachments in Outlook, files from untrusted locations, or files that were downloaded from the Internet.

While you could disable protected view to fix it, it’s not recommended as it can allow infected documents to run code.

The recommended solution is to fix the file permissions, or run Office's Online Repair.

To fix the file permissions:

Right click on the Start button and choose Command Prompt (Admin)
Copy icacls "%programfiles%\Microsoft Office 15" /grant *S-1-15-2-1:(OI)(CI)RX
Right click in the command prompt window and choose Paste.
Press Enter to run the command.

If you are unable to fix the file permissions, you’ll need to do an Online Repair of your Office installation from Control Panel, Program and Features. Note that Online Repair can take a long time to complete.

Right click on the Start button and choose Programs and Features (marked with an arrow in the screenshot above). If it’s not listed, choose Control Panel and search for programs.
Double click on Programs and Features when it comes into view.
Select your Microsoft Office install and choose Change from the menu above the program list.
Select Online Repair then click Repair to begin.