Hello Charles,
You can change the coding, but this is not supported by SAP.
There is an ABAP class, CL_HTTP_EXT_ITS, that can be accessed via SE24.
The MIMEs are stored in the application server file system, i.e. they are not available via SE80.
You can look at the $DIR_DATA\icmandir\its for the js and css files.
Note that you should not change these files, as every time you replace the kernel, the ICM process will extract the new MIMEs from the new patch.
Regards,
Cris