Fixed errcheck issues in server/channels/app/file_test.go (#28941)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/.golangci.yml

@@ -87,7 +87,6 @@ linters:
         path: "\
           channels/api4/apitestlib.go|\
           channels/api4/channel_test.go|\
-          channels/app/file_test.go|\
           channels/app/helper_test.go|\
           channels/app/platform/helper_test.go|\
           channels/store/localcachelayer/channel_layer.go|\
@@ -219,4 +218,4 @@ formatters:
       - mock.*
 issues:
   max-issues-per-linter: 0 # no maximum
-  max-same-issues: 0 # no maximum
+  max-same-issues: 0 # no maximum