When working in the field offline, it is unreasonable to expect users to go through and manually download the media associated with each record.
Having an option to 'also sync/download media up to X days old' might be one way of offering this.
Alternatively, allowing the media files to be manually transferred to the device into a known folder structure!
The use case I've encountered is where we have a historic 'Photo point monitoring' App in which users want to review previous photos while in the field.