Add support for human readable file types instead of mime_types for user facing messages.

Should be easier for non-programmers to understand.
Use if statements instead of ternary
human readable/human-readable
Better documentation for $human_file_types
Fix a comment typo
Further documentation.
fix spelling error in comments
Merge pull request #25 from isaacgrant/file-entry-human-readable-messages

Add support for human-readable file types instead of mime_types for user facing messages.