In one of my development with Telerik RadGrid, i experienced the following 2 issues:
1). The file you are trying to open, ‘Filename.xls’, is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening this file. Do you want to open the file now?
2). Format (Number) column values/ styling cell or row
Find below the Url from Telerik, you can get some more number of tips over there: