I just checked again. The gmail attachment issue no longer applies to the current release of SlimBrowser.
Bug: in gmail, when you click "attach a file" button and select a file to attach, nothing happens and the file is not attached. If you clear the cache, the uploading will work for the session. But it will fail again after restarting slimbrowser.
Workaround: click the "Settings" link to reach the settings page, turn on the option "basic attachment features", click "save changes".
The possible cause is that newest version of gmail uses adobe flash to display the uploading progress of attachment files. Somehow some features of adobe flash player don't work well inside SlimBrowser, which causes the failure of the attachment uploading. This might be due to some security strictions of flash running inside SlimBrowser. Any ideas leading to the solution of this bug will be appreciated!