Question : Can't open a .xlsx file when attached to an Outlook email

We are using Office 2007 thru a Citrix Presentation Server 4.5 environment.  
The Scenerio is:
A user creates an excel spreadsheet with the extension .xlsx, and saves it to her My Documents.  She then open up Outlook 2007, starts and email, and inserts this worksheet as an attachment.  She tries to open it up in the email before she sends it and it won't open.

Then she goes back and renames the worksheet using the .xls extension and goes thru the same procedure.  Now the attachment opens up just fine.   Any ideas on how to fix this so she doesn't have to rename her files when she wants to attach them?

Answer : Can't open a .xlsx file when attached to an Outlook email

Does this work from Explorer?

Right click on the file while it exists as an .xlsx and select 'Open With' - at the bottom of this window after selecting Microsoft Excel, there will be a button to 'always open with this program'. Highlight that and then press OK.

From now on, all .xlsx files should work by double clicking them.
I don't see why she needs to re-open them before attaching them though. She can just save her work, close excel. Attach file and send it. The other receiving party should have no problems opening it.