Server cannot set Content type HTTP header information after sending

Yesterday a user reflects, export Excel failed, I tested, the amount of data is small no problem, the amount of data is slightly larger will appear this problem. Consultation Degree Niang also did not find the right solution, suddenly think of the system, other modules have the ability to download Excel attachments, but never appeared this problem. So I referred to the relevant code to modify, although the problem solved, but the principle is not quite understand.

The specific solution is as follows:

Server cannot set Content type HTTP header information after sending