field2* converters should not handle files

[jugmac00]

Until now field converters could be applied on files with unclear outcome.

The problems with trying to convert files to text (detect MIME type, guess charset...)
were discussed in length in #558 (comment).

Handling files only makes sense for the field2bytes converter.

This fixes #558

[icemac]

I general I like the idea, i just have some questions resp. suggestions.

[d-maurer]

I do not think we need deprecation warnings in Zope 5: Zope 5 is Python 3 only and the file case is wrongly handled by the converters; thus, any user should already have been warned.

From my point of view, deprecation warnings are also not necessary in Zope 4: I assume that the "convert file" case is very rarely used.

[jugmac00]

@icemac and @d-maurer, thank you for taking the time for the review!