[MM-61099] Fix errcheck issues in server/channels/app/brand.go (#30679)

* [MM-28779] Fix errcheck issues in server/channels/app/brand.go

Remove brand.go from the errcheck exclusion list in .golangci.yml and fixed the error by properly handling the return value from a.MoveFile().

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* [MM-28779] Add test to verify brand image backup functionality

Add a new test that verifies backup of the original brand image happens when a new one is uploaded. This helps to ensure the fix for errcheck issues is working as expected.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* use seperate temporary filestore for each test

* Use FileSettings.Directory instead of finding the dir programatically

* Fix another test

* Fix defer

* Update server/channels/api4/job_test.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fix bad bot commit

* Cleanup logs message

* cleanup file path

* Fix error variable names

* WIP:cleanup panic ussage

* Revert "WIP:cleanup panic ussage"

This reverts commit c3284e4427a41c818acc161926cd2535dee9a6b9.

* cleanup error checks

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/brand_test.go

@@ -6,6 +6,7 @@ package api4
 import (
 	"context"
 	"net/http"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -64,6 +65,76 @@ func TestUploadBrandImage(t *testing.T) {
 	CheckCreatedStatus(t, resp)
 }
 
+func TestUploadBrandImageTwice(t *testing.T) {
+	th := Setup(t)
+	defer th.TearDown()
+
+	data, err := testutils.ReadTestFile("test.png")
+	require.NoError(t, err)
+
+	// First upload as system admin
+	resp, err := th.SystemAdminClient.UploadBrandImage(context.Background(), data)
+	require.NoError(t, err)
+	CheckCreatedStatus(t, resp)
+
+	// Verify the image exists and contents match what was uploaded
+	receivedImg, resp, err := th.SystemAdminClient.GetBrandImage(context.Background())
+	require.NoError(t, err)
+	require.NotNil(t, receivedImg)
+	require.Equal(t, http.StatusOK, resp.StatusCode)
+	require.NotEmpty(t, receivedImg, "Received image data should not be empty")
+
+	// Get the list of files in the brand directory
+	files, err := th.App.FileBackend().ListDirectory("brand/")
+	require.NoError(t, err)
+	require.Len(t, files, 1, "Expected only the original image file")
+
+	// ListDirectory returns paths with the directory prefix included
+	fileName := files[0]
+	fileName = strings.TrimPrefix(fileName, "brand/")
+	require.Equal(t, "image.png", fileName, "Expected the original image file")
+
+	// Second upload (which should back up the previous one)
+	data2, err := testutils.ReadTestFile("test.tiff")
+	require.NoError(t, err)
+
+	resp, err = th.SystemAdminClient.UploadBrandImage(context.Background(), data2)
+	require.NoError(t, err)
+	CheckCreatedStatus(t, resp)
+
+	// Get the list of files in the brand directory again
+	files, err = th.App.FileBackend().ListDirectory("brand/")
+	require.NoError(t, err)
+
+	// Should now have the new image.png and a backup with timestamp
+	require.Len(t, files, 2, "Expected the original and backup files")
+
+	// Check that one of the files is image.png
+	hasOriginal := false
+	hasBackup := false
+	for _, file := range files {
+		// ListDirectory returns paths with the directory prefix included
+		fileName := strings.TrimPrefix(file, "brand/")
+
+		if fileName == "image.png" {
+			hasOriginal = true
+		} else if strings.HasSuffix(fileName, ".png") && strings.Contains(fileName, "-") {
+			// Backup file should have a timestamp format like 2006-01-02T15:04:05.png
+			hasBackup = true
+		}
+	}
+
+	require.True(t, hasOriginal, "Original image.png file should exist")
+	require.True(t, hasBackup, "Backup image file should exist")
+
+	// Verify the new image is available through the API and matches what was uploaded
+	receivedImg2, resp, err := th.SystemAdminClient.GetBrandImage(context.Background())
+	require.NoError(t, err)
+	require.NotNil(t, receivedImg2)
+	require.Equal(t, http.StatusOK, resp.StatusCode)
+	require.NotEmpty(t, receivedImg2, "Received image data should not be empty")
+}
+
 func TestDeleteBrandImage(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()