I have managed to upload files of types not in the list of approved types. I have also had problems uploading files which are in the list of approved types.
for example if I select a file of the wrong type, I get the error message 'Please enter a value with a valid mimetype.'
(Users won't understand this message - can it be changed?)
However when I then try to select a file of correct file extension, it is not accepted, I get the same error.
On occasions I have had no trouble uploading files of invalid types.