[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/cmd/mmctl/commands/import_e2e_test.go

@@ -401,7 +401,7 @@ func (s *MmctlE2ETestSuite) TestImportValidateCmdF() {
 		s.Require().Equal(struct {
 			UnusedAttachments []string `json:"unused_attachments"`
 		}{
-			UnusedAttachments: []string{"data/test2.png"},
+			UnusedAttachments: []string{"data/test2.png", "data/test_img_diff_A.png", "data/test_img_diff_B.png"},
 		}, printer.GetLines()[1].(struct {
 			UnusedAttachments []string `json:"unused_attachments"`
 		}))