I am just looking for the most appropriate way to indicate with HTTP POST my content type.
I have csv files. They may have 1 line or they may have multiple lines. As I am implementing the server side of this, distinguishing between the two can make my implementation a bit cleaner.
text/cms+csv
Is what I have but that doesn't indicate how many entities exist in the payload. Do I use a custom param?
There is no requirement to indicate the number of entities in a request payload. If you are uploading multiple files in a single request, then each file in that request will have a corresponding entity. Each entity in the request would then include its content-type, etc. This StackOverflow post shows an example.
If I understand your case correctly, you are uploading a single csv which may or may not have multiple lines. There is no header that describes this. The closest header to your case is arguably the Content-Length header. This header indicates the size of an entity body.
To answer your question more directly, if you really need the line count, you might create a custom header. And as for an appropriate Content-Type, you should consider text/csv.
