We are not modifying it so there is no need to copy memory around.
This also prevents file_mime getting out of sync as had already
happend. It had a size of 50 while the mime types database type used
to have a size of 70.
This way the compiler can figure out the correct sizing of the strings
for us and we won't forget to increase the char arrays if a longer
mime type gets declared.
While here introduce nitems macro to simplify for loop.