I've encountered an issue when developing an API where I need to decide which Content-Type to utilize for POST requests. It's common knowledge that for uploading files through a web browser, the multipart/form-data Content-Type is a must. However, I find myself in a scenario where there's no browser involved, and it's strictly API communication between servers.
To give some context, I have endpoints that receive simple flat data, like strings and numbers, and others that potentially receive files or a mix of files and data. I'm trying to figure out if there's a performance impact, or any other reason that would guide the choice for one Content-Type over the other in a purely API context.
My question really boils down to: what are the considerations for using application/x-www-form-urlencoded vs multipart/form-data Content-Types in server-to-server communication where file upload may or may not be required?
An example request using application/x-www-form-urlencoded might look like this:


In contrast, here's an example using multipart/form-data, potentially with a file:


I'm looking for input based on technical specifications, performance analytics, or best practices that can influence the decision in such scenarios.

When it comes to API design, the choice between form-urlencoded and multipart concerns more than just file upload capabilities. form-urlencoded is fine for simple data but doesn't support binary data without encoding it, which adds overhead.
Multipart, on the other hand, is built for mixed content. It can handle both binary and text data, which makes it suitable for file uploads and combined data types. Each part of the message is separate, which means you could stream parts of the request if needed, potentially reducing memory overhead for large files.

Software_Expert, you're on the right track considering the format for your POST requests. Always keep in mind that form-urlencoded should essentially be used for submitting simple text fields. It's more efficient and straightforward for small, structured data payloads. But for anything heavier or more complex, such as files or non-ASCII content, multipart is the way to go.
Utilizing multipart also helps when dealing with non-file binary data. It gives you a clear structure for your payload and does not require additional encoding like base64, which you would need with form-urlencoded.

To add to what webInnovator and techwiz84 mentioned, also consider standardization within your API. If your endpoints might sometimes receive files and other times only text, it may simplify client implementations if you standardize on multipart/form-data for all endpoints. This way, clients won't need to adjust headers based on the type of data being sent, potentially reducing errors and client-side code complexity.
Using multipart when files are not involved does have a bit more overhead than form-urlencoded, due to the boundaries and headers for each part. However, this is often negligible unless you're operating at a very large scale or have a high-performance requirement.
Here's a refined version of the multipart code example, including the necessary imports and error handling:


Remember to have proper exception handling in place, as network requests can fail for various reasons, and it's crucial to close file handles after you're done with them to avoid resource leaks.