Summary
Adding a new filter to Media Manager's search requires two actions: adding a new filter to the "DigiZuite_System_Framework_Search" search; adding the same filter to the searches "DigiZuite_System_Framework_My_Favorites" and "DigiZuite_System_Framework_My_Videoes; creating a new label (if one does not already exist).
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Go to the DAM Center → System tools → ConfigManager → choose DigiZuite™ Media Manager → choose the version, which your Media Manager is set up to use → click the "Searches" tab → add an asset type filter → save the search → populate the search → ensure that the search has been populated by refreshing the page; if the "populate search cache" cog-wheel button is not present, the search has been successfully populated.
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Then you MUST add the same filter to the searches "DigiZuite_System_Framework_My_Favorites" and "DigiZuite_System_Framework_My_Videoes". If a label with the correct Constant and Label, then you MUST create one. Unlike Add new meta data field to Media Manager's Asset Preview, if a label with a Constant matching the added filter's ID does not exists, the Media Manager will throw an error, which results in the filters being unavailable.
In this case, a label already exists with a Constant "sAssetType" and Label "Asset type".
After doing so, refreshing Media Manager will display three filters:
Removing a filter
This is done by removing the filter from the searche "DigiZuite_System_Framework_Search".