Hi, just wondering, that the preview image of a field type “file” is always just a filetype icon.
maybe you can change current line 119 (ACF Pro 5.9.6) of “includes/fields/class-acf-field-file.php”. The variable $attachment already returns, whether it is an image file type or not. So there is no need, just to show an icon. It would be more helpful to show the image itself instead.