You don't seem to understand the issue.
When I write "Then the upload is just ignored. "A more appropriate response would be "The upload file does not have the extension .csv. The upload request has been ignored."", that is what I mean.
If you chose to ignore the upload when the extension is wrong - that's your call (or the programmer's), no objections there. (There are more flexible solutions around, but whether they are appropriate to use here, I don't know)
But what I humbly suggest is that when you ignore a user task, you should not JUST ignore it, you should ALSO tell the user what you did and feed back clues as to what happened and why and how to avoid it.
Simply so.