I would clone the dashboard (in this case, misc.xml), and then follow the instructions here in docs to add a form input to an existing dashboard. If you wanted to constrain the panels by a folder name for example, then that field is called "source.item_name". So, after adding a form field to make a new replacement token available to the panels, a search might look like this:
index=box source.item_type="file" AND (event_type="preview" OR event_type="download" OR event_type="upload") source.item_name=$folder_name$ | dedup event_id | stats count by event_type
Note the "$folder_name$" token has to match the name that you use in the corresponding form field.
If you end up doing much work in this direction, I would suggest starting over, and use the form to filter a base search. Then you would change all of the panels to use the base search, instead of having a simultaneous search for each panel against the same data. You would use searchPostProcess, or in 6.2, the new search element.
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