[MM-57500] Streaming support for importing file attachments (#26629)

* Bulk import: stream file attachments uploads

* Add comment with context on buffer size

* Add file name to logs

* Use sha256 to do checksum

* Fix bad merge

* Fix import file

* Update test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/file.go

@@ -633,6 +633,12 @@ func UploadFileSetRaw() func(t *UploadFileTask) {
 	}
 }
 
+func UploadFileSetExtractContent(value bool) func(t *UploadFileTask) {
+	return func(t *UploadFileTask) {
+		t.ExtractContent = value
+	}
+}
+
 type UploadFileTask struct {
 	Logger mlog.LoggerIFace
 
@@ -659,6 +665,10 @@ type UploadFileTask struct {
 	// the file.  Plugins are still invoked.
 	Raw bool
 
+	// Whether or not to extract file attachments content
+	// This is used by the bulk import process.
+	ExtractContent bool
+
 	//=============================================================
 	// Internal state
 
@@ -729,14 +739,15 @@ func (a *App) UploadFileX(c request.CTX, channelID, name string, input io.Reader
 	))
 
 	t := &UploadFileTask{
-		Logger:      c.Logger(),
-		ChannelId:   filepath.Base(channelID),
-		Name:        filepath.Base(name),
-		Input:       input,
-		maxFileSize: *a.Config().FileSettings.MaxFileSize,
-		maxImageRes: *a.Config().FileSettings.MaxImageResolution,
-		imgDecoder:  a.ch.imgDecoder,
-		imgEncoder:  a.ch.imgEncoder,
+		Logger:         c.Logger(),
+		ChannelId:      filepath.Base(channelID),
+		Name:           filepath.Base(name),
+		Input:          input,
+		maxFileSize:    *a.Config().FileSettings.MaxFileSize,
+		maxImageRes:    *a.Config().FileSettings.MaxImageResolution,
+		imgDecoder:     a.ch.imgDecoder,
+		imgEncoder:     a.ch.imgEncoder,
+		ExtractContent: true,
 	}
 	for _, o := range opts {
 		o(t)
@@ -803,7 +814,7 @@ func (a *App) UploadFileX(c request.CTX, channelID, name string, input io.Reader
 		}
 	}
 
-	if *a.Config().FileSettings.ExtractContent {
+	if *a.Config().FileSettings.ExtractContent && t.ExtractContent {
 		infoCopy := *t.fileinfo
 		a.Srv().GoBuffered(func() {
 			err := a.ExtractContentFromFileInfo(c, &infoCopy)
@@ -1264,15 +1275,6 @@ func (a *App) SetFileSearchableContent(rctx request.CTX, fileID string, data str
 	return nil
 }
 
-func (a *App) getFileInfoIgnoreCloudLimit(rctx request.CTX, fileID string) (*model.FileInfo, *model.AppError) {
-	fileInfo, appErr := a.Srv().getFileInfo(fileID)
-	if appErr == nil {
-		a.generateMiniPreview(rctx, fileInfo)
-	}
-
-	return fileInfo, appErr
-}
-
 func (a *App) GetFileInfos(rctx request.CTX, page, perPage int, opt *model.GetFileInfosOptions) ([]*model.FileInfo, *model.AppError) {
 	fileInfos, err := a.Srv().Store().FileInfo().GetWithOptions(page, perPage, opt)
 	if err != nil {
@@ -1317,20 +1319,6 @@ func (a *App) GetFile(rctx request.CTX, fileID string) ([]byte, *model.AppError)
 	return data, nil
 }
 
-func (a *App) getFileIgnoreCloudLimit(rctx request.CTX, fileID string) ([]byte, *model.AppError) {
-	info, err := a.getFileInfoIgnoreCloudLimit(rctx, fileID)
-	if err != nil {
-		return nil, err
-	}
-
-	data, err := a.ReadFile(info.Path)
-	if err != nil {
-		return nil, err
-	}
-
-	return data, nil
-}
-
 func (a *App) CopyFileInfos(rctx request.CTX, userID string, fileIDs []string) ([]string, *model.AppError) {
 	var newFileIds []string