Fix #23458 Batch conversion silently fails to support output as an array of files

[pacebes]

Resolves: #23458

This PR allow json batch conversion files including an array of output files such as:

[
    {
        "in":"Score_23458.mscz",
	     "out":[
			"Score_23458_4.mp3",
			"Score_23458_1.pdf",
			"Score_23458_2.pdf",
			"Score_23458_3.pdf"
			]
    }
]

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