Upload file directly to volume/filer with its mime-type instead of multipart/form-data and a single file entry

[KevinBLT]

Hello,

it would be really useful to just upload a single file as is without using multipart/form-data

So like these examples:
Curl:
curl -X POST --header 'Content-Type: application/pdf' --data @test.pdf http://filer:8080/test.pdf

JS:

import { readFile, stat } from 'node:fs/promises';

await fetch('http://filer:8080/test.pdf', {
  method: 'POST',
  body: await readFile('test.pdf'),
  duplex: 'half',
  headers: {
    'content-type': 'application/pdf',
    'content-length': (await stat('test.pdf')).size
  }
});

I think in some cases it's much easier to just send the file over and omit the process of creating boundary etc. for multipart/form-data.

I would imagine that in the corresponding go section it's an if/else block and would be fully backwards compatible.

If there was a go dev able to do this, I would appreciate :)

[KevinBLT]

See: #5539