Fix file upload through htmx.ajax

[Telroshan]

Fixes #2630

Description

When converting an object to FormData, we had a simple check that if a value was an object, we'd JSON-stringify it before appending it to the FormData.

#2748 addressed a similar issue, thoughI had totally missed that a similar check was done elsewhere in the code.

One case was not handled here, causing the referenced issue ; the Blob type, that FormData can perfectly handle (a type that includes Files), would be JSON-stringified instead of directly passed to FormData.append as it's an object itself, resulting in the loss of the file data, and that conversion to an empty object.

Corresponding issue: #2630

Testing

Added a test cases, to ensure that htmx.ajax with a file passed in parameters, is correctly uploaded and not empty.
Also added another test case with a file input as I found out we didn't have any yet.

Additionnaly, you can:

• Observe the current behavior on this JSFiddle, notice how the file is an empty object instead of a binary
• Observe the behavior with this PR's fix on this JSFiddle, notice how the file is now correctly a binary

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded