or making use of a regex like

# note not sure where the file extension is within param,
# I am assuming at end of string, anchoring the regex
# to the end will speed it up slightly
$regex = '\.(gif|jpg|jpe[g]?|jfif)(\s+|)$';

if ($filename !~ /$regex/o) {
   error("Your image needs to be in GIF or JPEG format.");
}
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/*
* And the Creator, against his better judgement, wrote man.c
*/