FileFormatQ   

gives True if the file corresponding to source might be imported as format "fmt" and gives False otherwise.

FileFormatQ[source,{"fmt₁","fmt₂",…}]

gives True if source might be imported as one of "fmt_i".

Details and Options

FileFormatQ attempts to identify whether a file can be imported as a specific format.
In FileFormatQ[source,…], source can be any of the following:
"file" or File["file"] file name, searched for on $Path

"url" or URL["url"] HTTP, HTTPS or FTP URL
FileFormatQ uses heuristic methods and may not give correct results, particularly for shorter files.
FileFormatQ does not verify that Import would succeed if applied to source.
FileFormatQ gets clues from both names of files and properties of files recorded in the filesystem, but bases the final result it returns only on the actual contents of files.
FileFormatQ[url] accepts the following URLDownload options:

Authentication	None	authentication information to send
CookieFunction	Automatic	function to apply to each cookie received
ConnectionSettings	Automatic	speed and other settings for the connection
FollowRedirects	True	whether to follow HTTP redirects
Interactive	True	whether to allow interactive authentication dialogs
TimeConstraint	Infinity	time to wait for a response
VerifySecurityCertificates	True	whether to verify SSL security certificates

open allclose all

Check if a file might be a JPEG:

Check another file:

Check if a file might be in one of the image formats:

Check if a URL might be a PDF:

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	"file" or File["file"]	file name, searched for on $Path
	"url" or URL["url"]	HTTP, HTTPS or FTP URL