Optimisation of work with plugins files. (0002064)

Fix the caching of plugin MIME file types

Update common types ($g_plugin_mime_types) used by plugin_file_include()

• Replace the old 'text/javascript' with 'application/javascript'
• Add modern SVG and WebP image types

Performance improvements:

• avoid loading the magic database if not needed
• Add 'Last-Modified' header and return 304 if unchanged

Fixes 0035199, PR https://github.com/mantisbt/mantisbt/pull/2064

Signed-off-by: Damien Regad dregad@mantisbt.org