ui_plain: automatically lookup mimetype when mimetype-file is set
For sites that do not want to configure mime types by hand but
still want the correct mime type for 'plain' blobs, configuring
a mime type file is made possible. This is handy since such a
file is normally already provided (at least on Linux systems).
Also, this reflects the gitweb option '$mimetypes_file'
Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl> Signed-off-by: Lars Hjemli <hjemli@gmail.com>