The issue seems to be that you can copy text from the excel file opened via the Mattermost app. We’re using react-native-file-viewer to preview file attachments. However, the copy/paste protection cannot be applied there.
Our devs responded that we cannot disable the share icon nor the functionality provided by it, as we are not able to do anything like that in a third party app for Android. The one thing you could do to prevent this is to disallow downloading files in the mobile app.