如何在python中检查上传的文件是csv还是xls ?

How to check is upload file is CSV or XLS . How to check it in python. I'm importing a file to a binary field in openerp which can be retrived as a binary object. I need to read the file and import the data to a table. User can upload csv or xls file. By knowing only I can use the csv package or xlrd package. . How