refactor ifs

[kmlebedev]

What problem are we solving?

small refactor #5539

• if to switch
• string metrod to const

How are we solving the problem?

improve readability

How is the PR tested?

Checks

• I have added unit tests if possible.
• I will add related wiki document changes and link to this PR after merging.

[chrislusf]

Go test failed.

[KevinBLT]

@kmlebedev

I would assume test fails because at least curl for example sends Content-Type: multipart/form-data; boundary="----xyz", so I think it's not possible to simply test with equality operator here.

Additionally, I am not sure if the content-type is always set to multipart/form-data by existing clients, so if it's empty I think it should be treated as multipart/form-data for backwards compatibility. 🙂