What you see is a compiler warning, but the app will run.
Your condition is always true:
contentType != "image/jpeg" || contentType != "image/png"
You compare a string variable to 2 different string values (using not equal), so one of them will surely be true, and true || false is always true.
Most likely you need logical AND: I assume you want to test if the content type is neither JPEG nor PNG:
if contentType != "image/jpeg" && contentType != "image/png" {
return
}