How do I show all files in a folder, not only 3D models?

Project Manager shows only the file types enabled in the filter, so nothing is lost — it is just hidden.

1. Click the file-type filter button on the panel.
2. On the Models tab, disable the filters to see every file. 
3. To also see files from nested folders, turn on Show files in subfolders in the tree settings.

To add a file type that isn’t in the list: How do I show or hide specific file types in the asset browser?