Back end designed an interface, send a condition, return a file, just beginning is no problem. Until one day this is operated, download returned Null file, next the newspaper of format of a Json in the file is wrong. Next I am checked, checked discovery a long time, this newspaper is wrong, the state code that back end returns is 200, but returned Json form (the unsuccessful format) of common interface, I here it is to think of course, as long as be 200 it is normal return. Downloaded so next back end raises this requirement, changeover enforces to answer text first when returning data namely, looking is Json and sign up for wrong code. If be to be cast,go out unusual but I think, mix below this kind of circumstance with (normal file is returned and sign up for a fault to return) , have a problem namely originally. After all the document that you do not know to the family is passed, in case how be in that way format does? Then requirement back end changes state code, but back end does not change. . . . . . . Then the problem was laid aside, ask old man so people the interface that how should design this download file, how should when making mistake, return. . .