You can access these settings in the Settings application under the Content -> Media category.
Use permanent URLs
If true, URLs of medial library files will be generated in permanent format (~/getmedia/<file guid>/<file name>.<file extension>.<files friendly URL extension>), otherwise direct path to the file will be used (e.g.: ~/MySite/Media/MyLibrary/MyImage.jpg).
Maximum number of folders that can be displayed under an expanded folder node in the tree view.
Media file allowed extensions
Extensions of files which can be uploaded to media libraries. Divide the extensions by semicolons.
Check files permissions
Indicates if the "See media library content" permission is checked when retrieving media files using permanent URLs.
Media libraries folder
Folder where media library files are stored. You can use:
If no value is entered, the default location is: ~/<site code name>/media
Note: When you change the location of the media library folder, the system does NOT automatically move existing media files and sub-folders to the new location. To ensure that your media content is accessible and works correctly, you need to manually move the files from the old location to the new one in the file system.
Use site-specific subfolders for custom media libraries folder
This setting is applied only when a Custom media libraries folder is configured. If enabled, media library files will be stored in a sub-folder named as the site code name under the custom files folder, i.e. <custom media libraries folder>/<site code name>. It is useful for better orientation in files when multiple sites are running in the system.
Media file hidden folder
Name of the folder where thumbnails of media files will be stored. This folder is hidden in the file system by default and thumbnails are generated within it.
Media file thumbnail suffix
Suffix added to thumbnail files. The thumbnail file names are in the following format:
<file name>_<file extension><thumbnail suffix>.<thumbnail extension>