MM-50419: Fix panic at interface conversion (#22284)

We were incorrectly returning a subset of the methods,
whereas at other places, clients needed access to other
interfaces as well.

https://mattermost.atlassian.net/browse/MM-50419

```release-note
NONE
```

shared/filestore/s3store.go

@@ -57,6 +57,11 @@ var (
 	imageMimeTypes  = map[string]string{".jpg": "image/jpeg", ".jpeg": "image/jpeg", ".gif": "image/gif", ".bmp": "image/bmp", ".png": "image/png", ".tiff": "image/tiff", ".tif": "image/tif"}
 )
 
+var (
+	// Ensure that the ReaderAt interface is implemented.
+	_ io.ReaderAt = (*s3WithCancel)(nil)
+)
+
 func isFileExtImage(ext string) bool {
 	ext = strings.ToLower(ext)
 	return imageExtensions[ext]
@@ -208,7 +213,7 @@ func (b *S3FileBackend) MakeBucket() error {
 // s3WithCancel is a wrapper struct which cancels the context
 // when the object is closed.
 type s3WithCancel struct {
-	io.ReadSeekCloser
+	*s3.Object
 	timer  *time.Timer
 	cancel context.CancelFunc
 }
@@ -216,7 +221,7 @@ type s3WithCancel struct {
 func (sc *s3WithCancel) Close() error {
 	sc.timer.Stop()
 	sc.cancel()
-	return sc.ReadSeekCloser.Close()
+	return sc.Object.Close()
 }
 
 // CancelTimeout attempts to cancel the timeout for this reader. It allows calling
@@ -237,7 +242,7 @@ func (b *S3FileBackend) Reader(path string) (ReadCloseSeeker, error) {
 	}
 
 	sc := &s3WithCancel{
-		ReadSeekCloser: minioObject,
+		Object: minioObject,
 		timer:  time.AfterFunc(b.timeout, cancel),
 		cancel: cancel,
 	}

shared/filestore/s3store_test.go

@@ -10,7 +10,6 @@ import (
 	"encoding/base64"
 	"errors"
 	"fmt"
-	"io"
 	"net/http/httptest"
 	"net/http/httputil"
 	"net/url"
@@ -19,6 +18,7 @@ import (
 	"testing"
 	"time"
 
+	s3 "github.com/minio/minio-go/v7"
 	"github.com/stretchr/testify/require"
 )
 
@@ -283,20 +283,25 @@ func TestS3WithCancel(t *testing.T) {
 	})
 }
 
-func newMockS3WithCancel(timeout time.Duration, closeErr error) (*s3WithCancel, context.Context) {
+func newMockS3WithCancel(timeout time.Duration, closeErr error) (*fauxCloser, context.Context) {
 	ctx, cancel := context.WithCancel(context.Background())
-	return &s3WithCancel{
+	return &fauxCloser{
-		ReadSeekCloser: fauxCloser{strings.NewReader("testdata"), closeErr},
+		s3WithCancel: &s3WithCancel{
+			Object: &s3.Object{},
 			timer:  time.AfterFunc(timeout, cancel),
 			cancel: cancel,
+		},
+		closeErr: closeErr,
 	}, ctx
 }
 
 type fauxCloser struct {
-	io.ReadSeeker
+	*s3WithCancel
 	closeErr error
 }
 
 func (fc fauxCloser) Close() error {
+	fc.s3WithCancel.timer.Stop()
+	fc.s3WithCancel.cancel()
 	return fc.closeErr
 }