Fix model artifact upload form type bug

What does this MR do and why?

Fix model artifact upload bug

• When uploading a file, the PUT endpoint expects a single body with a single file as content.
• The UI model file uploads sends a multiform data; which is wrong. This will be so, in the future but creates a problem.

Model registry feature is in beta testing; and we shall communicate the current beta users.

Followup: Add a feature and/or QA spec for ML model file ... (#477841)

Buggy behavior

When we upload a file called file.txt with contents

Foo

We download it and we get:

------WebKitFormBoundarya3lu19v6obLLbaq9
Content-Disposition: form-data; name="file"; filename="file.txt"
Content-Type: text/plain

Foo
------WebKitFormBoundarya3lu19v6obLLbaq9--

When running your branch on GDK and repeating the same steps I am able to download the file with the original content.

MR acceptance checklist

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Screenshots or screen recordings

Upload and download a file to see all works fine.

How to set up and validate locally

• Go to a project URL in your browser
• Open the "Deploy" and "Model registry" under it.
• Create a model and a version.
• Upload some files
• Download them and see if all works fine.