I spent some time struggling with configuring my server to allow filesendr to accept uploads > 10MB in size. There is a document in the documentation area titled 'Configuring your Domino server to accept large file uploads' which discusses the changes to the 'Maximum POST size' field in the internet site document. However, there is another configuration change that you might need to make. In the server document, the internet protocols>http>'maximum size of request content' field also needs to be increased past its 10 MB default.
It would be helpful if the second field were included in the official documentation. Thanks for considering this request!