When working with files from the local file system, you are going to have to write some special-purpose code for the web client. Remember that the web client session is running on the server. To the web client session, "local" means on the server machine. When a user selects a file with the file dialog, they're selecting a path on their local machine. The server doesn't have access to that file. The solution is to transfer the file from the client to the server, where the file can be accessed from a temporary location for the session.
If you look at Chapter 43 of the Dynamics GP Integration Guide (IG.pdf) that is included with Dexterity, you'll find the last section of the chapter is titled "Working with the local file system". It contains all of the details you should need to get your code working correctly for the web client.
You can find the same information n the IG.chm file if you'd rather look for it their. That help file is also installed with Dexterity.