diff --git a/solarfarmer/api.py b/solarfarmer/api.py
index 586cb96..e322c69 100644
--- a/solarfarmer/api.py
+++ b/solarfarmer/api.py
@@ -231,7 +231,9 @@ def _make_request(
         timeout = self._get_timeout(params)
         headers = {
             "Authorization": f"Bearer {key}",
-            "ContentType": "application/json",
+            # Content-Type intentionally omitted — requests sets this automatically
+            # based on whether files are present (multipart) or not (form-encoded).
+            # Forcing it here overrides the boundary parameter and breaks multipart uploads.
             "User-Agent": "solarfarmer-api-sdk/" + __version__,
         }