First, this is a question first posted in the Outlook forum. It has been cross-posted at this thread and this one. No satisfactory answer has come up, I suspect that it is because this is a side-effect of Win 10 rather than Outlook or Word settings.
Originally posted in Outlook forum on November 25, 2015:
I have multiple versions of Office on my computer but use the latest version of Outlook (2013) that I have.
When I double-click on a Word document in Windows it opens in Word 2010, which is my preferred version. However, if I open a Word document that is an email attachment directly, it opens in Word 2013.
Is there any setting I can change to get attachments to open in Word 2010.
BTW, I do know that it is not a good idea to open and edit Word documents directly from email. I generally open them and if I want to edit, I save the document to a permanent storage location.
Note, this is a recent behavior (within the last couple of days). Before that attachments opened in Word 2010.
Additional information: This was not a problem for more than two years under Windows 8.1. This is true of all forms of Word files.
I also tried a command line re-registration, following my own directions from: My docs open in the wrong program! Re-registering Word using the commandline.
Here is a screenshot from a message showing the .docx attachments with Word 2013 icons.
Here is a screenshot of such an attachment's icon when downloaded/saved to the Desktop using SaveAs. That document also showed a Word 2013 icon as the attachment.
Note that it is the Word 2010 icon.
When a document is opened from a file saved to the computer it opens in Word 2010. When the same document is opened directly from the email, it opens in Word 2013. This happens whether or not Word 2010 or Word 2013 are already open. AFAIK this only happens when the email is opened in Outlook 2013.
Again, this is a relatively recent development. I have been using Outlook (and Word) 2013 for more than two years previously without this anomaly. Word 2010 was and remains the default application for Word documents, except when they are opened directly from Outlook.
Word 2013 is currently not the default app for any file type.