[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/import_test.go

@@ -267,12 +267,16 @@ func TestImportProcessImportDataFileVersionLine(t *testing.T) {
 }
 
 func GetAttachments(userID string, th *TestHelper, t *testing.T) []*model.FileInfo {
+	t.Helper()
+
 	fileInfos, err := th.App.Srv().Store().FileInfo().GetForUser(userID)
 	require.NoError(t, err)
 	return fileInfos
 }
 
 func AssertFileIdsInPost(files []*model.FileInfo, th *TestHelper, t *testing.T) {
+	t.Helper()
+
 	postID := files[0].PostId
 	require.NotNil(t, postID)