mimedetect 5.x-1.0

Changes since DRUPAL-5--1-0-beta: Module review, resulting mostly in style improvements, but also some "real" changes

• Use the 'mimedetect_file_binary' variable instead of 'filefield_file_binary'.
• Use a $t() instead of t() in the requirements function.
• Merge the last two mime type checks.
• Move the declaration of the $mime variable further down for better readability, and remove an unnecessary check on !$mime.
• Add the mime type for png, and fix ico and rgb (was 'rbg' before).
• Tune user visible strings for sounding "official" and consistent casing.
• Remove trailing spaces.
• strip charsets from text mime types returned by the UNIX 'file' command.