The file manager, when opened from an image content, filters the results to avoid the selection of a non image file (for example, avoid that a PDF is selected to replace an image).
To link a file you can:
Select an image or a button and use the Link file action bellow the URL field
Select a text, use the insert link dialog, and select "File manager" from the options drop-down
In both cases, the file manager will open showing all the available files.
Now you can upload a new file or select an existing one.
The link will be created once you indicate the file to use.