Edited Unassigned: extenderControlBase file missing [27819]

Hi,
I have recently upgraded from a website project to a web application project in visual studio.
I am using VS 2013.

Since then my confirm button extenders are not working as the project is requesting the ExtenderControlBase file and cannot find it.

I have reinstalled the AjaxToolKit (I am using the latest version), and it did not solve the problem.

I have also checked that the AjaxMin.dll is in the references folder of my project.
It's not in the bin folder however.

I have also checked the Property of the solution, to add the file in the debug source file, but I cannot find the file on my computer. I have managed to get the file, but if I add it there it does not solve the problem. Unless I am not adding it properly.

I don't know what else to do.

Thank you for help in advance

Anne