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Except for the html document, currently all the files (css, javascript and images) served from my web server do not specify the content-type in the http response header. I happen to notice this as I intend to remove the type attribute in both my script tag and link tag, but came across this answer which says the following:

The MIME type is also sent via the HTTP Content-Type header, so using type="text/css" would only be extra bytes.

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I believe you need to specify Content-Type for scripts to execute in IE. This is to prevent malicious users uploading scripts to image sharing websites. –  Brendon Dec 3 '13 at 10:53
@Brendon, thanks for pointing out the security implication. Is there any other aspect worth noting, security-wise, assuming no user uploaded content? –  Question Overflow Dec 3 '13 at 11:07
Are you sure your server is not sending a Content-Type header for these files? If you are viewing the response from files served from your browser cache then you probably won't see a Content-Type header. –  w3d Dec 3 '13 at 11:59
Most servers will send the correct mime types by default, so unless you explicitly set your server to not send it, I'm betting the files are being sent with the correct mime type. –  DisgruntledGoat Dec 3 '13 at 17:58
@w3d, thanks for your comment. I realize after rechecking that what I am reading off is actually from the request header of a 304 not modified file. Not exactly the cache, but the content-type is dropped from the header. Disabling the cache, everything appears normal again. –  Question Overflow Dec 4 '13 at 1:39
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1 Answer

up vote 4 down vote accepted

By the HTTP protocol, clause 7.2.1, “Any HTTP/1.1 message containing an entity-body SHOULD include a Content-Type header field defining the media type of that body. If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource.”

So yes, the response headers should contain Content-Type header for any response data (called “entity-body” in the protocol, often “file” in common language). If it is omitted, the browser is allowed to make its own wild guesses on the type of data it got. In many contexts, the risk of wrong guesses is negligible, but this not a good excuse for violating the protocol.

Attributes like type=text/css and type=text/javascript have not been necessary, except by some formal specifications, or even useful. Even if a server incorrectly sends e.g. CSS data without Content-Type, browsers will treat the data as CSS if <link rel=stylesheet ...> was the element that caused the request. It’s the rel attribute that matters in that case.

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Thanks for quoting the specs. This is helpful. –  Question Overflow Dec 3 '13 at 11:03
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