Dear Marie,
Datawarehouse only has CSV and TDE file formats.
Mostly Excel detect the CSV file to text the columns. If not, please use the below.
It is about how you delimiting the comma's in Excel Editor(Hope you are using Microsoft Excel).
1. Open CSV File in Excel.
2. Select the column(Only one column now right?) and click "Text To Columns"

3. Click on "Delimited" and click next.
4. In Delimiters, check "Others" and type comma ",".

5. Click "Finish" to see the columns populating correctly.
If you are using other editors than MS Excel, I dont have any idea.
Thank You!
Arun