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I routinely use Excel for TDT and other formats. I've never had a problem getting Windows to open .tdt files in Excel by default; in Windows 7, it's right-click | Open with | Choose default program... | Browse | C:\Program Files\Microsoft Office\Office##\Excel.exe | Open.

However, I recently installed Excel 2013, and the above technique no longer works. Even after navigating to Excel and hitting Open, Excel does not appear in the list of programs.

So I guess I need to add the association in the registry somewhere. I see the list of extensions in HKCR, but the entry for CSV implies that Excel has special handling for that extension; anyway, copying the keys and value from .csv to .tdt has no impact. I also see HKCU\Software\Classes.tdt, with a "Content Type" of "application/", but that doesn't seem to be doing anything.

Finally, there's HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts.tdt\OpenWithList. There's a value for EXCEL.EXE and several others, but only two of these (7-Zip and Notepad++) appear in the GUI. I can choose either of these two apps. However, it doesn't seem to have changed the content of either registry key, so there's no obvious entry to change.

Googling around, I see plenty of discussions about using the GUI, but the registry doesn't come up as often, and no two seem to be the same.

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Did you try to right-click the TDT file, click "Properties", and under the General Tab where it says "Open with:", click the "Change". Browse to the location in Program Files > Office > Office[x] > excel.exe

That should work.

I would also try changing it to something else (Notepad, for example), reboot, and then change back to Excel - see if that works.

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This should work. I wouldn't bother messing with the registry entries. Actually, I'd recommend against it for OP unless he really knows what he's doing. – David Zemens Feb 19 '13 at 20:29
No, the Properties tab doesn't work any better than the right-click | Open with menu. I've rebooted since setting it to Notepad++, with no change. – Jon of All Trades Feb 19 '13 at 22:08
Did it work when you set it to Notepad++? What about after setting it back to Excel and then rebooting again? – Joshua Feb 20 '13 at 14:59
It works with Notepad++, that is files now open in Notepad++ by default. The issue is that I can't set the default to Excel, as it does not appear in the menu even after browsing to the actual .exe. – Jon of All Trades Feb 20 '13 at 18:44

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