You cannot find and select Community files in the 'Add Links to Files' dialog box when creating Activity entries. You know the files exist because they can be found by search. The option "Attach File" only allows you to browse and upload a file from your system. How can you share a community file in an activity entry?
To link a Community file or a file uploaded into the Files application within an activity entry, use one of the following options:
Add Custom Fields > Link to File
Within an Entry or a To Do item, select Add Custom Fields > Link to File to be able to browse and select from Community files and files in the application. Note that a Comment does not have this particular option.
You can also use a bookmark to provide a means to access the file.
1. Open Community Activity (which needs to be created in advance)
2. Open and edit Entry/To Do/Comments
3. Click Add Bookmark
4. Enter the name of the community file you want to register.
5. Enter the Community File URL in URL field.
6. Click Save.
Within the rich text for an item in the Activity, you can add the URL link to the file.