[Feature Request] Add option to customize boundary in multipart request

[Tienisto]

Hello,

I would like to provide a custom boundary for the multipart request.
It seems that it is currently hard coded to generate a random string:

reqwest/src/async_impl/multipart.rs

Line 291 in 5715050

boundary: gen_boundary(),

I would like to provide a custom boundary like this:

let mut form = reqwest::multipart::Form::new();
form = form.boundary("-----------------custom boundary");

[ovnicraft]

@Tienisto as you mention gen_boundary() generates a random value

reqwest/src/async_impl/multipart.rs

Line 515 in 5715050

fn gen_boundary() -> String {

looks easy to add it, can you gives more info about why do yo need it?

[Tienisto]

I am developing an API client. I can imagine that users might want to provide a custom boundary to debug or test things out.
I think this is quite niche though.

[ovnicraft]

I'll open a PR and let's see if it works for you.

[seanmonstar]

Is there an actual use case needed, or is it currently only that some users might want it?

I don't feel particularly inclined to add it for a hypothetical...

[Tienisto]

Well, as I mentioned, I am developing an API client (similar to Postman or Insomnia) so I would like to have this feature (that's why I created this issue). If there is a hypothetical security vulnerability when setting a specific boundary, then users might want to try it out without digging into hyper or switching to Python.

Besides that, what is the purpose of exposing the boundary in the first place if we assume that it is a low-level concern? Adding a setter next to the getter would seem logical.

[ovnicraft]

also would like to know what the http spec said about it?