linux - How do you change the MIME type of a file from the terminal?

Question

What I'm looking for is a counterpart to file -I (Darwin; -i on Linux).

For example, given:

$ file -I filename.pdf
filename.pdf: application/octet-stream; charset=binary

I would like to be able to do something like this:

$ [someCommand] filename.pdf application/pdf

The result would be that filename.pdf would then be typed as application/pdf.

The reason for the question is that sometimes web servers use the wrong MIME type, which results in programs refusing to open the file. (Most often text/plain, in my experience.)

I've been searching man, the web and this site for about two and a half hours. Tried everything from hex dumps to xattr to text editors.

Your help would very much be appreciated.

Chris

Answer 1

The thing about MIME types is they're almost entirely fictional.

MIME and HTTP ask us to pretend that all of our files have a piece of metadata identifying the "content type". When we send files around the network, the "content type" metadata goes with them, so nobody ever misinterprets the content of a file.

The truth is this metadata doesn't exist. By the time MIME was invented, it was really too late to convince any OS vendors to adopt a new type system for files. Unix had settled on magic numbers, DOS had settled on 3-letter filename suffixes, and classic MacOS had its creator codes and type codes. (MacOS type codes were closest to the MIME model, since they actually were separate from both the filename and the content. But being only 4 letters long, MIME types wouldn't fit.)

Nobody stores MIME-compatible content types in their filesystem. When a MIME message composer or HTTP server wants to send a file, it decides the file type in the traditional way (filename suffix and/or magic number) and maps the result to a MIME type.

In contrast to the theory (where MIME eliminates file type guessing), MIME as implemented in practice has moved the "guess file type based on filename suffix and/or magic number" logic from the receiver of the file to the sender. As you have noticed, the sender doesn't usually do a better job than the receiver would have done if forced to figure it out for itself. Frequently in the case of a web server, the server's eagerness to slap a Content-type on a file makes things worse. There's no reason for a web server to know anything about the format of files it serves when it is only being used to distribute them and has no need to interpret their contents.

The file command guesses file type by reading the content and looking for magic numbers and strings. The -I option doesn't change that. It just chooses a different output format.

To change the Content-Type header that a web server sends for a specific file, you should be looking in your web server's configuration manual. There's nothing you can do to the file itself.