[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/i18n/en.json

@@ -5410,13 +5410,17 @@
     "id": "app.import.attachment.bad_file.error",
     "translation": "Error reading the file at: \"{{.FilePath}}\""
   },
+  {
+    "id": "app.import.attachment.file_stat.error",
+    "translation": "Error reading the file status: \"{{.FilePath}}\""
+  },
   {
     "id": "app.import.attachment.file_upload.error",
     "translation": "Error uploading the file: \"{{.FilePath}}\""
   },
   {
-    "id": "app.import.attachment.read_file_data.error",
-    "translation": "Failed to read file attachment during import."
+    "id": "app.import.attachment.seek_file.error",
+    "translation": "Error seeking the file: \"{{.FilePath}}\""
   },
   {
     "id": "app.import.bulk_import.file_scan.error",