Steps to reproduce:
1. Navigate to my documents;
2. Upload a file through the browser;
3. Upload the same file on the mobile application;
The file is not uploaded and no error message is shown.
When user tries to create a folder with the same name of one already on server, the message "A folder with same name already exists on server" is displayed.
So an equivalent message should be displayed when the user tries to upload a file with the same name of one already on server.