Top menu >> Setup >> Files

Check your file usage and trash files to free up space if needed. Only users with Setup privilege will be able to see this page. 

From the Setup page click Files

On this page you can exactly see how many active files you have and what your current limit is. 

If you want you can trash files using the Trash button in the last column on the right of your page. You can sort by any column by clicking on the column header to find the files you would like to trash.