Media endpoint currently returns an empty list for filtered and merged datasets

[FrankApiyo]

Environmental Information

• Onadata version: v3.8.3

Problem description

• A request to the media endpoint (for merged datasets and filtered datasets) incorrectly yields an empty list for datasets with image attachments
• Example:

  $ curl -H "Authorization: TempToken <your temp token ;-)>" https://stage-api.ona.io/api/v1/media.json?xform=323&type=image&page_size=10&page=1
  $ []

Expected behavior

Requests to the media endpoint for merged datasets and filtered datasets that have media file attachments should yield the attachments

Steps to reproduce the behavior

• Create a merged dataset or filtered dataset that contains media files
• Make a request to the media/attachments endpoint to fetch the media files it contains