HttpHandlers / Modules :: A Potentially Dangerous Request / Path Value Was Detected From The Client (?)

I am using Webhandler to upload images to the server. I want to send the folder name so on that folder the images will save. I am using this URI format and got the below error.

builder.Path = builder.Path.TrimEnd('/')
+ "/Services/FileReceiver.ashx?foldername=" +
folder;
this.Uri
= builder.Uri;

Also I added the following line in the web.config but still having the issue.

<httpRuntime requestValidationMode="2.0" />

A potentially dangerous Request.Path value was detected from the client (?). Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: A potentially dangerous Request.Path value was detected from the client (?).

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack
Trace:
[HttpException (0x80004005): A potentially dangerous Request.Path value was detected from the client (?).]
System.Web.HttpRequest.ValidateInputIfRequiredByConfig() +8884233
System.Web.ValidateRequestExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +35
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184

Version

Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.1