Description:
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if no call is made to header() , php inserts Content-Type: text/html

this may well be incorrect, as for example in creating image/png

HTTP spec allows sending content without a content-type header and allows the client to detect type from the data, in this case first four bytes are %PNG

However, this content sniffing is allowed "if and only if" there are no headers explicitly present.

Thus in adding a default text (or other inappropriate default) header php is preventing the client from correctly displaying the data.



Reproduce code:
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<?php
$i=imagecreate(....);
imagepng($i);
imagedestroy($i);
?>

Expected result:
----------------
data served without content-type header and correctly interpreted by client sniffing content as defined in spec.

Actual result:
--------------
incorrect insertion of erroneous header forces image data to be displayed as text.