You must customize these settings. The default settings are too permissive for regulated industries (Finance, Healthcare, Legal) and too restrictive for engineering firms that rely on legacy CAD-to-Excel exports.
The actual error depends on the Office version, but the fix is always the same: The IT admin must either unblock that file type globally, or the user must use a third-party tool to convert the file to a modern format. Look closely at the File Block Settings dialog. For each file type, there is a third option nested in the dropdown: "Open selected file types in Protected View" (instead of blocking them outright). file block settings in the trust center
In modern Microsoft 365 Apps (Semi-Annual Enterprise Channel), that override is often removed. If you block a file type, it is blocked . The only way to open it is for an admin to change the Trust Center policy or temporarily move the file to a whitelisted location (which is not a real fix). The Migration Strategy: How to Phase Out Legacy Formats If you want to finally kill .doc in your organization, do not flip the "Hard Block" switch tomorrow. That is a riot waiting to happen. Use a 3-phase strategy: You must customize these settings
We often talk about macros, add-ins, and ActiveX controls when discussing Office security. But lurking just a few clicks away in the Trust Center is a feature that is simultaneously one of the most protective and one of the most frustrating in the Microsoft 365 ecosystem: File Block Settings . Look closely at the File Block Settings dialog
| File Type | Extension | Risk Level | Recommended Action | | :--- | :--- | :--- | :--- | | | .xlm | Critical | Hard Block (Open & Save) | | Word 2 / Word 6.0 | .doc (pre-97) | High | Hard Block | | Excel 95 Workbooks | .xls (pre-97) | High | Hard Block | | PowerPoint 95 | .ppt (pre-97) | Medium | Protected View | | Web Pages | .htm , .html | Medium | Block Open (they trigger scripts) | Group Policy: Managing at Scale The worst way to manage File Block Settings is by walking to each desk. The best way is via Group Policy Administrative Templates (ADMX/ADML).