Sometimes we might be asked to serve up a zero-length input file,
typically from some machine-generated CSS etc. We make some very
rudimentary guess about the content-type the caller wanted and skip the
mmap(2) call.
If a .svg file doesn't have a “<?xml…” prolog, then it would be detected as plain
text. Allow extension-based override of the content-type in this case.
Laurence Withers