Hello,
What do you mean by the attachment changes to Word viewer?
Provide us the screen shot of the exact changes of the attachment?
You may download the file and perform the file association for Word and check if it helps.
Right click on downloaded attachment
Word file/ Open with and Choose Default Program/ Select Word
And ensure to check the box “always use this program to open this kind of file” and click on
OK.
Once you complete the above mentioned steps, you may open Word attachment and check if it helps.
If the above steps fail, you may repair Office 2013 using the following link and verify the status:
http://office.microsoft.com/en-us/excel-help/repair-or-remove-office-2010-HA010357402.aspx
I hope the above information helps. Let us know if you need further assistance.
Thank you.