@@ -8,7 +8,6 @@
 
 import requests
 import requests.utils
-from requests_toolbelt.multipart.encoder import MultipartEncoder  # type: ignore
 
 import gitlab
 import gitlab.config
@@ -637,38 +636,6 @@ def _check_redirects(result: requests.Response) -> None:
                 )
             )
 
-    @staticmethod
-    def _prepare_send_data(
-        files: Optional[Dict[str, Any]] = None,
-        post_data: Optional[Union[Dict[str, Any], bytes]] = None,
-        raw: bool = False,
-    ) -> Tuple[
-        Optional[Union[Dict[str, Any], bytes]],
-        Optional[Union[Dict[str, Any], MultipartEncoder]],
-        str,
-    ]:
-        if files:
-            if post_data is None:
-                post_data = {}
-            else:
-                # booleans does not exists for data (neither for MultipartEncoder):
-                # cast to string int to avoid: 'bool' object has no attribute 'encode'
-                if TYPE_CHECKING:
-                    assert isinstance(post_data, dict)
-                for k, v in post_data.items():
-                    if isinstance(v, bool):
-                        post_data[k] = str(int(v))
-            post_data["file"] = files.get("file")
-            post_data["avatar"] = files.get("avatar")
-
-            data = MultipartEncoder(post_data)
-            return (None, data, data.content_type)
-
-        if raw and post_data:
-            return (None, post_data, "application/octet-stream")
-
-        return (post_data, None, "application/json")
-
     def http_request(
         self,
         verb: str,
@@ -746,7 +713,9 @@ def http_request(
             retry_transient_errors = self.retry_transient_errors
 
         # We need to deal with json vs. data when uploading files
-        json, data, content_type = self._prepare_send_data(files, post_data, raw)
+        json, data, content_type = self.http_backend.prepare_send_data(
+            files, post_data, raw
+        )
         opts["headers"]["Content-type"] = content_type
 
         cur_retries = 0
 
@@ -1,4 +1,4 @@
-from typing import Any, Dict, Optional, Union
+from typing import Any, Dict, Optional, Tuple, TYPE_CHECKING, Union
 
 import requests
 from requests_toolbelt.multipart.encoder import MultipartEncoder  # type: ignore
@@ -21,6 +21,38 @@ def __init__(self, session: Optional[requests.Session] = None) -> None:
     def client(self) -> requests.Session:
         return self._client
 
+    @staticmethod
+    def prepare_send_data(
+        files: Optional[Dict[str, Any]] = None,
+        post_data: Optional[Union[Dict[str, Any], bytes]] = None,
+        raw: bool = False,
+    ) -> Tuple[
+        Optional[Union[Dict[str, Any], bytes]],
+        Optional[Union[Dict[str, Any], MultipartEncoder]],
+        str,
+    ]:
+        if files:
+            if post_data is None:
+                post_data = {}
+            else:
+                # booleans does not exists for data (neither for MultipartEncoder):
+                # cast to string int to avoid: 'bool' object has no attribute 'encode'
+                if TYPE_CHECKING:
+                    assert isinstance(post_data, dict)
+                for k, v in post_data.items():
+                    if isinstance(v, bool):
+                        post_data[k] = str(int(v))
+            post_data["file"] = files.get("file")
+            post_data["avatar"] = files.get("avatar")
+
+            data = MultipartEncoder(post_data)
+            return (None, data, data.content_type)
+
+        if raw and post_data:
+            return (None, post_data, "application/octet-stream")
+
+        return (post_data, None, "application/json")
+
     def http_request(
         self,
         method: str,