correctly encode content uploads as multipart/form-data

[evgeni]

before this, they were encoded as application/x-www-form-urlencoded, and
while this technically works, it makes the request 2-3× larger,
sometimes hitting limits in Rack/Rails.

Fixes: #1862

[evgeni]

interestingly, the uploaded data is corrupt 🙈

[evgeni]

https://projects.theforeman.org/issues/38482 - Katello/katello#11408

[evgeni]

As this seems to require more work, let's apply a workaround for now: #1872

[ekohl]

@evgeni now that Katello/katello#11408 is fixed, can this progress?

[evgeni]

Technically, yes.
It'd make the code incompatible with older releases (unless we do some wild if/else based on version), so I've been putting it off for now.
Is there any pressure to get this in?

[ekohl]

I came across this in https://redhat.atlassian.net/browse/SAT-41766 which mentioned there was only a workaround in place. But you're right that not breaking compatibility with versions prior to Katello 4.18 is also a valuable thing.