diff --git a/extensions/mattermost/src/mattermost/monitor.ts b/extensions/mattermost/src/mattermost/monitor.ts
index 7de24cb03e6..f603ee2bb37 100644
--- a/extensions/mattermost/src/mattermost/monitor.ts
+++ b/extensions/mattermost/src/mattermost/monitor.ts
@@ -743,7 +743,7 @@ export async function monitorMattermostProvider(opts: MonitorMattermostOpts = {}
         out.push({
           path: saved.path,
           contentType,
-          kind: core.media.mediaKindFromMime(contentType),
+          kind: core.media.mediaKindFromMime(contentType) ?? "unknown",
         });
       } catch (err) {
         logger.debug?.(`mattermost: failed to download file ${fileId}: ${String(err)}`);