Fix wording in info message.

edit.maxfilesize is an internal detail that has no meaning to
the user, and it makes for broken grammar.
Respect max_file_size set to 0 to mean "unlimited"

As documented, an explicitly set value of 0 should mean "any size".

Also, consistently initialize the value to 100K, as documented.

See Redmine 6588
Merge pull request #2001 from vohi/redmine/6588

Redmine/6588