actix_multipart: Incorrect field content type defaults

[jacob-pro]

Current Behavior

The Field::content_type() method returns a default of application/octet-stream in the case that it is not present

https://github.com/actix/actix-web/blob/bd5c0af0a6bf9a720cf0685b3334151de1d43c32/actix-multipart/src/server.rs#L364-L368

Expected Behavior

According to the multipart specification:

Each part MAY have an (optional) "Content-Type" header field, which defaults to "text/plain". If the contents of a file are to be sent, the file data SHOULD be labeled with an appropriate media type, if known, or "application/octet-stream".

So in the case of a text part, then we should actually get a default of "text/plain" and not "application/octet-stream".

Possible Solution

Unfortunately there is no simple way to provide a default, since from the server's perspective it is impossible to tell if the field is a "file", this is because the filename parameter is optional even for files:

For form data that represents the content of a file, a name for the file SHOULD be supplied as well, by using a "filename" parameter of the Content-Disposition header field. The file name isn't mandatory for cases where the file name isn't available or is meaningless or private; this might result, for example, when selection or drag-and- drop is used or when the form data content is streamed directly from a device.

Therefore if we want to provide reliable information to library users, the best thing we can do is make Field::content_type() return Option<&Mime> instead, to reflect the true content type that was set by the uploader.

[fakeshadow]

content-type should be optional for sure.

[jacob-pro]

Ok cool, once I've finished working on https://github.com/actix/actix-web/pull/2883 (which currently has a work-around) - then I will open a PR to fix this properly