fix: 502 on .flv/unknown-extension upload — use guess_type + guard None

ParamValidationError on copy_object after upload — ContentType=None
hit S3. get_content_type("thumbnail2") returned None for a .flv file
because mimetypes.types_map only holds Python's built-in table (no
.flv, .mkv, .opus, .m4v, etc.).

Switch to mimetypes.guess_type, which initializes from the OS mime
database and covers every common format. Keep None as the return for
truly unknown extensions so downstream callers that chain `or
"video/mp4"` still pick the right context-aware fallback.

Guard the two copy_object / mirror_key_async sites in views/product.py
with `or "application/octet-stream"` so even a genuinely unknown
extension can't crash the upload handler again.

make_post_sell/models/product.py

@@ -418,10 +418,17 @@ class Product(RBase, Base):
         return file_key
 
     def get_content_type(self, file_key):
-        """return the content type string for the S3 object, or None."""
+        """Return the content type string for the S3 object, or None.
+
+        Uses mimetypes.guess_type which initializes from the OS mime
+        database — types_map alone only knows Python's built-in table
+        (no .flv, .mkv, .opus, etc.) and returns None for those.
+        """
         extension = self.extensions.get(file_key)
-        if extension:
-            return mimetypes.types_map.get("." + extension)
+        if not extension:
+            return None
+        content_type, _ = mimetypes.guess_type(f"x.{extension}")
+        return content_type
 
     def get_content_disposition(self, file_key):
         """

make_post_sell/views/product.py

@@ -356,6 +356,7 @@ def product_edit(request):
 
         # copy upload to our system defined s3 location.
         # https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/s3.html#S3.Client.copy_object
+        content_type = product.get_content_type(file_key) or "application/octet-stream"
         request.shop_uploads_client.copy_object(
             ACL=acl,
             Bucket=request.shop_bucket_name,
@@ -364,7 +365,7 @@ def product_edit(request):
                 "Key": s3_webhook_key,
             },
             ContentDisposition=product.get_content_disposition(file_key),
-            ContentType=product.get_content_type(file_key),
+            ContentType=content_type,
             CacheControl=cache_control,
             Key=f"{product.s3_path}/{file_key}",
             MetadataDirective="REPLACE",
@@ -377,7 +378,7 @@ def product_edit(request):
             request.shop_bucket_name,
             f"{product.s3_path}/{file_key}",
             product.shop,
-            content_type=product.get_content_type(file_key),
+            content_type=content_type,
             cache_control=cache_control,
         )