Testing file uploads with Carrierwave

Hi,

I want to write an application which enables a user to upload a PDF file
to our server.

I would like to develop this application using a test-driven approach
and the Rails default test suite.

However, I am not sure what I need to test and what I don't need to
test.

To elaborate: a user should be able to click on the "select file"
button, select a file and press "submit". If the file is a PDF then it
will be uploaded to the server and written to the filesystem (as opposed
to a database), keeping its original name.
If the file is not a PDF, then an error message will be shown and the
form will be re-rendered.

I would be really grateful if someone could give me some pointers as to
what to test here and what not.

For example, would it be sensible to test that when a file is selected
and the form is submitted, that a file is actually uploaded? Or is this
something which would have been tested by the developpers of
Carrierwave?

Thanks in advance.