I don't know if this helps but the following tells what the type of the image 
file is.  (In my case I copied a jpg to img1 and a png to img2, without 
extensions of course.)

userdata = userdata or { }
function userdata.mytype(s)
img1 is \ctxlua{userdata.mytype("img1")}\par
img2 is \ctxlua{userdata.mytype("img2")}



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