Actually, in Sitefinity 3.6 we have already addressed this problem.
In case you are using sf404.aspx approach for extensionless URLs, Sitefinity adds sf404.aspx extension to the value of the form action attribute in the page output.
For example, if you request the following URL
you should have the following HTML in the page View Source:
Then, if you do a postback, the page will be requested with the following URL:
Alternatively, instead of sf404.aspx method, you can use wildcard mapping to accomplish extensionless URLs.
Hope this is helpful.
the Telerik team