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 11.1 - 3 Render DAM for Sitecore Field Types section 3.5 and here DFS 11.1 - 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: Penguins-50051.jpg

I want to prefix all download names with the tag DFS.

<setting name="DFS.MediaPipeline.downloadFilename.mask" value="DFS-{stampvalue-asset:name}" />

Output: DFS-Penguins.jpg

I want the file to contain the photograph name, in this example is has the metafieldId 50055.

<setting name="DFS.MediaPipeline.downloadFilename.mask" value="Download-{stampvalue-asset:name}-{stampvalue-asset:50055}" />

Output: Penguins-John doe.jpg