Some aspects of the File Manager TAB are obvious, like the filters, but others are not ...

The file manager has a column called "assigned" there you will see 2 numbers with an icon,
-- one number shows how many items use the file inside "an image field"
-- and the other number how many items use the file inside a "file field"

CURRENTLY it does not show the usage of files by minigallery or other third party fields

NOTE: You should never delete images manually, because they may be in use by items. First edit items to remove all uses of a file and then you can select it in the file manager and click delete.

FUTURE enhancements / plans:

1. Listing the usage of file by minigallery fields
2. Add links to display the list of items that use each file
3. Allow editing of file data like names, description, and replacing the uploaded file
4. Since some files are not reused, e.g. images of galleries, allow such files to be used by fields without creating an entry in the filemanager