Efficiently check if a File object is a valid Image

Question

I have a directory of images that might not have downloaded, and need to delete the duds.
This is too slow: Import /* ImageQ /@ fileNames
In bash, I use the file command to inspect the metadata of an image file's header, but ImageQ only works on in-memory images not paths or File objects.
Is there some way to do this efficiently, without importing them?  There's a function for this I forgot.
Update:
FileFormat has a problem - it checks all known filetypes! e.g. you can't just check if the file is one of a list {"JPEG","PNG","TIFF"}.

Mr Puh · Answer

Maybe this helps you already:
Lets take the filenames as a provided list as you suggest.
fileNames = {} (*your list here*)

First, lets time the code you provided:
AbsoluteTiming[Import /* ImageQ /@ fileNames]

{0.204095, {True, True, True, True}}

Speed up by a factor of 2 gives this code:
AbsoluteTiming[MemberQ[Import[#, "Elements"], "Image"] & /@ fileNames]

{0.0822613, {True, True, True, True}}

Speed up by a factor of 20 seems to give this:
Extensions = {‰PNG, ‰JPG} (*or other extensions*)
AbsoluteTiming[MemberQ[Extensions, First@ReadList[#, Expression, 1]] & /@ fileNames]

{0.00788099, {True, True, True, True}}

Note that the last code also works if you delete the .png extension from the file name. So "image.png" and "image" give both True.

Efficiently check if a File object is a valid Image

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