feat: update new apis for file conversion

Signed-off-by: Elizabeth Danzberger <[email protected]>

Author: elzody

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_31.rst

@@ -55,7 +55,8 @@ to check whether the current Files app instance is a public share or not, and if
 Added APIs
 ^^^^^^^^^^
 
-- TBD
+- If an app supports file conversion, it may now register an ``\OCP\Files\Conversion\ConversionProvider`` which will
+  be called automatically based on the supported MIME types. An app may register as many of these as needed.
 
 Changed APIs
 ^^^^^^^^^^^^