What are all the possible values for HTTP "Content-Type" header?
You can find every content type here: http://www.iana.org/assignments/media-types/media-types.xhtml
The most common type are:
Type application
application/java-archive application/EDI-X12 application/EDIFACT application/javascript application/octet-stream application/ogg application/pdf application/xhtml+xml application/x-shockwave-flash application/json application/ld+json application/xml application/zip application/x-www-form-urlencoded
Type audio
audio/mpeg audio/x-ms-wma audio/vnd.rn-realaudio audio/x-wav
Type image
image/gif image/jpeg image/png image/tiff image/vnd.microsoft.icon image/x-icon image/vnd.djvu image/svg+xml
Type multipart
multipart/mixed multipart/alternative multipart/related (using by MHTML (HTML mail).) multipart/form-data
Type text
text/css text/csv text/html text/javascript (obsolete) text/plain text/xml
Type video
video/mpeg video/mp4 video/quicktime video/x-ms-wmv video/x-msvideo video/x-flv video/webm
Type vnd :
application/vnd.android.package-archive application/vnd.oasis.opendocument.text application/vnd.oasis.opendocument.spreadsheet application/vnd.oasis.opendocument.presentation application/vnd.oasis.opendocument.graphics application/vnd.ms-excel application/vnd.openxmlformats-officedocument.spreadsheetml.sheet application/vnd.ms-powerpoint application/vnd.openxmlformats-officedocument.presentationml.presentation application/msword application/vnd.openxmlformats-officedocument.wordprocessingml.document application/vnd.mozilla.xul+xml
I would aim at covering a subset of possible "Content-type" values, you question seems to focus on identifying known content types.
@Jeroen RFC 1341 reference is great, but for an fairly exhaustive list IANA keeps a web page of officially registered media types here.
As is defined in RFC 1341:
In the Extended BNF notation of RFC 822, a Content-Type header field value is defined as follows:
Content-Type := type "/" subtype *[";" parameter]
type := "application" / "audio" / "image" / "message" / "multipart" / "text" / "video" / x-token
x-token := < The two characters "X-" followed, with no intervening white space, by any token >
subtype := token
parameter := attribute "=" value
attribute := token
value := token / quoted-string
token := 1*<any CHAR except SPACE, CTLs, or tspecials>
tspecials := "(" / ")" / "<" / ">" / "@" ; Must be in / "," / ";" / ":" / "" / <"> ; quoted-string, / "/" / "[" / "]" / "?" / "." ; to use within / "=" ; parameter values
And a list of known MIME types that can follow it (or, as Joe remarks, the IANA source).
As you can see the list is way too big for you to validate against all of them. What you can do is validate against the general format and the type
attribute to make sure that is correct (the set of options is small) and just assume that what follows it is correct (and of course catch any exceptions you might encounter when you put it to actual use).
Also note the comment above:
If another primary type is to be used for any reason, it must be given a name starting with "X-" to indicate its non-standard status and to avoid any potential conflict with a future official name.
You'll notice that a lot of HTTP requests/responses include an X-
header of some sort which are self defined, keep this in mind when validating the types.