[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/post.go

@@ -1782,17 +1782,6 @@ func (a *App) GetFileInfosForPost(rctx request.CTX, postID string, fromMaster bo
 	return fileInfos, firstInaccessibleFileTime, nil
 }
 
-func (a *App) getFileInfosForPostIgnoreCloudLimit(rctx request.CTX, postID string, fromMaster bool, includeDeleted bool) ([]*model.FileInfo, *model.AppError) {
-	fileInfos, err := a.Srv().Store().FileInfo().GetForPost(postID, fromMaster, includeDeleted, true)
-	if err != nil {
-		return nil, model.NewAppError("getFileInfosForPostIgnoreCloudLimit", "app.file_info.get_for_post.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
-	}
-
-	a.generateMiniPreviewForInfos(rctx, fileInfos)
-
-	return fileInfos, nil
-}
-
 func (a *App) PostWithProxyAddedToImageURLs(post *model.Post) *model.Post {
 	if f := a.ImageProxyAdder(); f != nil {
 		return post.WithRewrittenImageURLs(f)