MM-43828: Pass object length for some image operations (#20711)

For user profile and plugin upload, we use a bytes.Buffer.
In that case, we know the object size and can find
it out from the length of the buffer.

This helps reduce multi-part uploads.

This approach can also be taken in thumbnail and preview
images. However, they use an io.Pipe to directly upload
the image as it is being encoded. We could make the whole
process in separate parts of writing the full image in the
buffer and then upload it. But taking a conservative approach
for now.

Also, while here, removed some unused code.

https://mattermost.atlassian.net/browse/MM-43828

```release-note
NONE
```

app/file.go

@@ -12,7 +12,6 @@ import (
 	"fmt"
 	"image"
 	"io"
-	"mime/multipart"
 	"net/http"
 	"net/url"
 	"os"
@@ -453,76 +452,6 @@ func GeneratePublicLinkHash(fileID, salt string) string {
 	return base64.RawURLEncoding.EncodeToString(hash.Sum(nil))
 }
 
-func (a *App) UploadMultipartFiles(c *request.Context, teamID string, channelID string, userID string, fileHeaders []*multipart.FileHeader, clientIds []string, now time.Time) (*model.FileUploadResponse, *model.AppError) {
-	files := make([]io.ReadCloser, len(fileHeaders))
-	filenames := make([]string, len(fileHeaders))
-
-	for i, fileHeader := range fileHeaders {
-		file, fileErr := fileHeader.Open()
-		if fileErr != nil {
-			return nil, model.NewAppError("UploadFiles", "api.file.upload_file.read_request.app_error",
-				map[string]any{"Filename": fileHeader.Filename}, fileErr.Error(), http.StatusBadRequest)
-		}
-
-		// Will be closed after UploadFiles returns
-		defer file.Close()
-
-		files[i] = file
-		filenames[i] = fileHeader.Filename
-	}
-
-	return a.UploadFiles(c, teamID, channelID, userID, files, filenames, clientIds, now)
-}
-
-// Uploads some files to the given team and channel as the given user. files and filenames should have
-// the same length. clientIds should either not be provided or have the same length as files and filenames.
-// The provided files should be closed by the caller so that they are not leaked.
-func (a *App) UploadFiles(c *request.Context, teamID string, channelID string, userID string, files []io.ReadCloser, filenames []string, clientIds []string, now time.Time) (*model.FileUploadResponse, *model.AppError) {
-	if *a.Config().FileSettings.DriverName == "" {
-		return nil, model.NewAppError("UploadFiles", "api.file.upload_file.storage.app_error", nil, "", http.StatusNotImplemented)
-	}
-
-	if len(filenames) != len(files) || (len(clientIds) > 0 && len(clientIds) != len(files)) {
-		return nil, model.NewAppError("UploadFiles", "api.file.upload_file.incorrect_number_of_files.app_error", nil, "", http.StatusBadRequest)
-	}
-
-	resStruct := &model.FileUploadResponse{
-		FileInfos: []*model.FileInfo{},
-		ClientIds: []string{},
-	}
-
-	previewPathList := []string{}
-	thumbnailPathList := []string{}
-	imageDataList := [][]byte{}
-
-	for i, file := range files {
-		buf := bytes.NewBuffer(nil)
-		io.Copy(buf, file)
-		data := buf.Bytes()
-
-		info, data, err := a.DoUploadFileExpectModification(c, now, teamID, channelID, userID, filenames[i], data)
-		if err != nil {
-			return nil, err
-		}
-
-		if info.PreviewPath != "" || info.ThumbnailPath != "" {
-			previewPathList = append(previewPathList, info.PreviewPath)
-			thumbnailPathList = append(thumbnailPathList, info.ThumbnailPath)
-			imageDataList = append(imageDataList, data)
-		}
-
-		resStruct.FileInfos = append(resStruct.FileInfos, info)
-
-		if len(clientIds) > 0 {
-			resStruct.ClientIds = append(resStruct.ClientIds, clientIds[i])
-		}
-	}
-
-	a.HandleImages(previewPathList, thumbnailPathList, imageDataList)
-
-	return resStruct, nil
-}
-
 // UploadFile uploads a single file in form of a completely constructed byte array for a channel.
 func (a *App) UploadFile(c *request.Context, data []byte, channelID string, filename string) (*model.FileInfo, *model.AppError) {
 	_, err := a.GetChannel(c, channelID)