Fix two small bugs with the RestAPI Datasource

[snowe2010]

Proposed Changes

First bug:

Spring automatically transforms binary content to base64, therefore the
logic in the RestApiExecutor was causing the data to be encoded twice,
causing much confusion and frustration for several weeks actually.

Second bug:

the fileUtils#saveDataAsFile function was not actually setting the type
correctly when attempting to convert the base64 data to a binary blob,
thus the files would always download as a 'dms' file and it would
never actually be openable. This also caused a lot of confusion. Both of
these bugs together made it impossible to tell why stuff was broken.

Combined this means that anytime you were trying to download binary data
you would never actually be able to open the file, as it would be only
base64 data inside of it.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

I tested by creating a repo with a test release and an xlsx file in it. https://github.com/snowe2010/test-release-downloads/releases/tag/v1

I then wrote a small app with two queries, one to retrieve asset from github, the other to download the asset to my computer.

I tested both with and without my change to each file. Both are necessary to get the file download working.

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[FalkWolsky]

Thank you very much for this valuable contribution and bug fix!
This will help surely a lot of Lowcoders!