PublicShow -- Determine mime-type for a file

Simple library to guess the mime-type from the extension of a file. As various applications need to do this type ofinferencing it seems worthwhile to place this functionality in an extensible library.

To be done
- Consider content handling (using the Unix file command)
- Allow parameters? (e.g. text/html; charset=UTF-8)
Source file_mime_type(+FileName, -MimeType) is semidet
True when MimeType is the mime-type to be used for sending FileName. The default rules can be overridden and extended using the hook mime_extension/2.
MimeType- is a compound term of the form Type/SubType.