Don't try and return it as a FilePathResult,
there's no need to overcomplicate things here. Firstly, put your HTM file inside a folder in your project, something like Files
.
Now, add the following to your web.config to prevent unauthorised access to your file:
<location path="Files">
<system.web>
<authorization>
<deny users="?" />
</authorization>
</system.web>
</location>
And then you can link to it by doing:
<img src="~/Images/question_frame.png"
style="margin-top:3px;height:18px;width:20px;"
onclick="window.open('/Files/Default.htm', 'NMCHelp',toolbar=no, scrollbars=yes, resizable=yes, top=50, left=50,width=750, height=600');"/>
While you're at it, you might want to think about removing those inline styles and use a class name and the inline JavaScript too.
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