I don't know which JAR has it, but I'll tell you how to find it using a tool. It's very useful for any project that has lots of confusing JAR dependencies.
The tool is
JBoss's
Tattletale.
It's a simple command line tool.
Once you've downloaded it, run it from a terminal:
java -jar tattletale.jar <sourcedir> [<outputdir>]
<sourcedir> is where all JARs are present. They can be in any subdirectory at any level, because the tool walks the entire directory tree of <sourcedir>
<outputdir> is where a HTML report is generated. After the tool finishes, open index.html in a browser and open the "Class Location" report.
Then just search for "Mimetype" on that page to find its JAR.