When requesting an download form DAM of Sitecore media handler, the default file will be the assets title (name).extension. The setting can be found in the file: App_Config\Include\damforsitecore\DFS.Settings.config and look for the parameter setting DFS.MediaPipeline.downloadFilename.mask.
<setting name="DFS.MediaPipeline.downloadFilename.mask" value="{stampvalue-asset:name}" />
The possible stampvalue tags can be found here DFS 9.2.0 - 3 Render DAM for Sitecore Field Types, section 3.5 and DFS 9.2.0 - 4 Accessing metadata through API.
Examples:
I want the file to contain title and selected quality id.
<setting name="DFS.MediaPipeline.downloadFilename.mask" value="{stampvalue-asset:name}-{stampvalue-selected:quality}" />Output : Penguinws-50051.jpg
I want to prefix all download names.
<setting name="DFS.MediaPipeline.downloadFilename.mask" value="Download-{stampvalue-asset:name}" />