[MM-28923] Fix errcheck issues in server/channels/app/file.go (#30878)

Co-authored-by: Claude <noreply@anthropic.com>

server/.golangci.yml

@@ -90,7 +90,6 @@ issues:
         channels/api4/websocket_test.go|\
         channels/app/bot_test.go|\
         channels/app/brand.go|\
-        channels/app/file.go|\
         channels/app/file_test.go|\
         channels/app/helper_test.go|\
         channels/app/permissions_test.go|\

server/channels/app/file.go

@@ -1165,7 +1165,9 @@ func prepareImage(rctx request.CTX, imgDecoder *imaging.Decoder, imgData io.Read
 	if err != nil {
 		return nil, "", nil, fmt.Errorf("prepareImage: failed to decode image: %w", err)
 	}
-	imgData.Seek(0, io.SeekStart)
+	if _, err = imgData.Seek(0, io.SeekStart); err != nil {
+		return nil, "", nil, fmt.Errorf("prepareImage: failed to seek image data: %w", err)
+	}
 
 	// Flip the image to be upright
 	orientation, err := imaging.GetImageOrientation(imgData, imgType)
@@ -1409,7 +1411,7 @@ func (a *App) CreateZipFileAndAddFiles(fileBackend filestore.FileBackend, fileDa
 	// Create Zip File (temporarily stored on disk)
 	conglomerateZipFile, err := os.Create(zipFileName)
 	if err != nil {
-		return err
+		return fmt.Errorf("failed to create temporary zip file %q: %w", zipFileName, err)
 	}
 	defer os.Remove(zipFileName)
 
@@ -1422,10 +1424,13 @@ func (a *App) CreateZipFileAndAddFiles(fileBackend filestore.FileBackend, fileDa
 		return err
 	}
 
-	conglomerateZipFile.Seek(0, 0)
+	_, err = conglomerateZipFile.Seek(0, 0)
+	if err != nil {
+		return fmt.Errorf("failed to seek to beginning of zip file %q: %w", conglomerateZipFile.Name(), err)
+	}
 	_, err = fileBackend.WriteFile(conglomerateZipFile, path.Join(directory, zipFileName))
 	if err != nil {
-		return err
+		return fmt.Errorf("failed to write zip file to file backend at path %s: %w", path.Join(directory, zipFileName), err)
 	}
 
 	return nil

server/channels/app/file_test.go

@@ -308,7 +308,7 @@ func TestCreateZipFileAndAddFiles(t *testing.T) {
 		err := th.App.CreateZipFileAndAddFiles(&mockBackend, []model.FileData{}, zipName, directory)
 
 		require.Error(t, err)
-		require.Equal(t, err.Error(), "only those who dare to fail greatly can ever achieve greatly")
+		require.Equal(t, err.Error(), "failed to write zip file to file backend at path directory-to-heaven/zip-file-name-to-heaven.zip: only those who dare to fail greatly can ever achieve greatly")
 	})
 
 	t.Run("write no file", func(t *testing.T) {