There is a iOS.PList.UIFileSharingEnabled build property but no one that leads to a LSSupportsOpeningDocumentsInPlace plist entry. This additional one is needed for displaying files in the new iOS files app (see UIFileSharingEnabled).
Sorry, I overlooked this Build settings:
Creating a uxl file with this content:
<Extensions Backend="CPlusPlus" Condition="iOS">
<Require Xcode.Plist.Element>
<![CDATA[
<key>UIFileSharingEnabled</key>
<string>YES</string>
<key>LSSupportsOpeningDocumentsInPlace</key>
<string>YES</string>
]]>
</Require>
</Extensions>
will also create the necessary plist entries.