Upload Widget Async.Save Handler Doesn't Account for Numeric-only String Returns (ASP.NET MVC)

[Jesse]

My web application's implementation of the Upload widget does data validation through the component itself (max file size, allowed file types, etc.) as well as some extra validation through the Async.Save() handler's controller action (image validity, checking for animated images, etc.). It's my understanding that there are three potential results from the handler:

1. Return of an empty string results in success.
2. Return of a Json string or object also results in success.
3. Return of anything else results in failure.

During the non-client side validation in my controller, should one of the checks fail we call "return Content(<string>)" and for the most part, this works as intended if the string contains any non-numeric characters. Initially I was simply returning a reference to a resource string defining the error and displaying it to the user in an alert describing what went wrong with the upload (via XMLHttpRequest.responseText); however, in the case of an undefined (by me) error this would result in an alert showing server response details that I don't wish to be displayed to the user. Thus I decided to send error codes and translate those client-side. This made it much easier to check for the error messages I mean to display to the user as well as have a default that displays for any unintended response. My problem is that if I return a string that is strictly numeric the handler doesn't result in an error.

Ex. return Content("Error: Bad File") would result in my error handler being called, and the text "Error: Bad File" being displayed in an alert.

But return Content("2") would result in a "successful" upload. The amount of numbers in the string doesn't matter, but if at any point a letter or symbol is included in the string it once again properly handles as an error.

I've already fixed my code to work by adding "error" into the string with the code number but I figured it was worth asking about since I would prefer to just use a numeric string. I'm puzzled as to what is causing the issue, as I don't believe strings containing only numbers are treated any differently than others unless they're being parsed. If I check the network response in the dev. console it shows the string as the response, which is what leads me to think it's something with the kendo widget. 

Using .NET Framework 4.7.2, ASP.NET MVC 4 and Kendo.Mvc version 2021.2.616.545.