The file manager, when opened from 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.