how to check file param exists ?

[wzhsh90]

var formData = new FormData();
formData.append("picFile1", narrowFiles[0]);
formData.append("picFile2", narrowFiles[1]);

formData.append("pub_time", "xxxx");

the picFile1 and picFile2 is Multipart file type.
the pub_time is text

how can i check picFile1 org picFile2 is exists and how can i get pub_time filed value ?

the multipart example does not show how to get it .

[wzhsh90]

<input type="file" name="picFile1"/>
<input type="file" name="picFile2"/>
<input type="text" name="pub_time"/>