If you are trying to download to excel from an SAP list viewer button, the ECC6.0 system may be trying to download to a default MHTML file format:
To change this, follow these steps: Select a transaction that shows a list viewer grid. There are many. One of the easiest is VA05. Position the cursor on the line and right click. Then press Spreadsheet
The following pop-up will appear and the default may be as shown below: