Doc: add a disclaimer discouraging multipart file uploads (#5293)

Co-authored-by: Maria Elisabeth Schreiber <[email protected]>

docs/source/advanced/upload.mdx

@@ -2,7 +2,12 @@
 title: Uploading files
 ---
 
-Apollo Kotlin supports file uploads via the [GraphQL multipart request specification](https://github.com/jaydenseric/graphql-multipart-request-spec).
+Apollo Kotlin supports file uploads via the [GraphQL multipart request specification](https://github.com/jaydenseric/graphql-multipart-request-spec) with a few caveats:
+
+- Uploading files with GraphQL is most often suitable for proof-of-concept applications. In production, using purpose-built tools for file uploads may be preferable. Refer to this [blog post](https://www.apollographql.com/blog/backend/file-uploads/file-upload-best-practices/) for the advantages and disadvantages of multiple approaches.
+- The [Apollo Router](/router/) doesn't support `multipart-form` uploads.
+
+## Uploading files with Apollo Kotlin
 
 First, add the following custom scalar mapping to the Apollo Gradle plugin configuration: