MM-40818: Pass filestore to channels (#19677)

We move away from referencing server struct
but pass the filestore service to Channels
explicitly.

https://mattermost.atlassian.net/browse/MM-40818

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/channels.go

@@ -17,6 +17,7 @@ import (
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/plugin"
 	"github.com/mattermost/mattermost-server/v6/services/imageproxy"
+	"github.com/mattermost/mattermost-server/v6/shared/filestore"
 	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 	"github.com/pkg/errors"
 )
@@ -48,6 +49,7 @@ type namer interface {
 type Channels struct {
 	srv        *Server
 	cfgSvc     configSvc
+	filestore  filestore.FileBackend
 	licenseSvc licenseSvc
 
 	postActionCookieSecret []byte
@@ -115,6 +117,7 @@ func NewChannels(s *Server, services map[ServiceKey]interface{}) (*Channels, err
 	requiredServices := []ServiceKey{
 		ConfigKey,
 		LicenseKey,
+		FilestoreKey,
 	}
 	for _, svcKey := range requiredServices {
 		svc, ok := services[svcKey]
@@ -133,6 +136,12 @@ func NewChannels(s *Server, services map[ServiceKey]interface{}) (*Channels, err
 				return nil, errors.New("Config service does not contain Name method")
 			}
 			ch.cfgSvc = cfgSvc
+		case FilestoreKey:
+			filestore, ok := svc.(filestore.FileBackend)
+			if !ok {
+				return nil, errors.New("Filestore service did not satisfy FileBackend interface")
+			}
+			ch.filestore = filestore
 		case LicenseKey:
 			svc, ok := svc.(licenseSvc)
 			if !ok {