We are trying to use our Office Online Server to render .mht files (Version 16.0.8471.8525).

We are testing this from the server URL directly for simplicity.

The error we receive when trying to render a new link created for an .mht file is the following. enter image description here

Though this error seems straighforward, the ULS Logs (C:\ProgramData\Microsoft\OfficeWebApps\Data\Logs\ULS), we see the following error:

Open From Url                   atd1q   Medium      File extension not on allow list: .mht

So, is there an "Allow" list somewhere where this can be enabled? I'm not sure to believe the error message or the logs here.


1 Answer 1


Office Online supports the following file types.

Supported files for viewing:

Word files: .doc, .dot, .dotx, .docx, .docm, .odt

Powerpoint: .pot, .potm, .pps, .ppsm, .ppt, .pptm, .pptx, ppsx, .odp

Excel .xls, .xlsx, .xlsm, .xlsb, .ods

Supported files for viewing, editing and annotating:

Word: .docx, .docm, .odt

Excel: .xlsx, .xlsm, .xlsb, .ods

Powerpoint: .pptx, ppsx, .odp

More on this :



  • I appreciate the response. However, we're working with a vendor on this, and they were able to render it fine in their environment with the same version. The official Microsoft documentation also only states those are the defaults, and not whether there is a way to deviate from those standards. I'm inclined to believe there is some option for this. Feb 19, 2020 at 16:30

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