There is a bug with Internet Explorer where it seems to inaccurately identify an Office 2007/2010/2012 documents (DOCX, XLSX, PPTX) as a ZIP file and then uncompresses it to its base XML format. This causes the system to want to open that file in a XML editor like Dreamweaver.
The instructions below are specific to Word DOCX files, but it is the same for all office "x" extensions including XLSX and PPTX.
Options to get around this issue:
The Sender - Save the with a different extension. Either save it as a DOC file or add a fake extension (.doug) and ask the receiver to remove the extra extension when saving the file upon receipt/download.
The Receiver
Option 1 - Save the file and make sure the file extension is DOCX when saving it, even if you have to change the extension (last three/four letters). You may have to save it by whatever name it comes with and then rename it afterwards with the "DOCX" extension.
Option 2 - Ask the sender to email the file with a different extension. The simplest way is to ask them to save it as a DOC file instead of a DOCX. Or you can ask them to add a fake extension (.doug) to the document and then when you save the file, simply remove the .doug from the end of the file.
Option 3 - Try using a different application other then Internet Explorer to read/download the file. Firefox is a very good free browser that works well and does not have the same issue with these files.
Option 4 – Read your email in a different application. If you are reading your webmail application, then download the email to your local email application instead or wait till you are back in the office to read the message.