Remove file_target_extension option as redundant

[michalfita]

Problem

All board files define "file_target_extension" option that except one case¹ is equal to the filename of the downloaded image. That option feeds Extension member of StepDownload, however, looking into the code shows that this is supposed to append an extension to downloaded file without one, what should never be the case. Even the Jetson's image loads attachement with .zip extension, so it can always be derived.

I personally failed at this option couple times dealing with my board files (that aren't and can't be public).

Resolution

Removal or making this optional (I don't think we need this complexity) would make use of this plugin easier for users.

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¹ - that file is jetson-nano/ubuntu.json

[michalfita]

Note: looking at the code looks like this option is already redundant and optional. Needs some experiments.