Bader:
I cannot reproduce your failure. I am able to upload an .mht file using the following code. See the attached image ("radeditor_documentmanager_upload.png") for the results. I used your server-side code to set the search pattern.
Default.aspx:
Default.aspx.cs:
The selected upload is "Toolbars.mht" and it is 1.27 mb.
Have you tried uploading smaller .mht files to see if this relates to an upload timeout issue? Are you able to successfully upload other file types such as .zip or .rar files that are 4.5mb in size?
Cheers!