Add extensions to assets in the full media export

[Kit Blake]

When doing a full media export, all assets are assigned a number as the file
name. But since there is no file extension, they're not very usable if you open
the zip to extract, say, all the images. The export code can include the
extension to make this more friendly.

There is an issue with backwards compatibility, but if the code disregards the
extension this should be possible.

[Eric Casteleijn (thisfred)]

Done in 1.6. Please test a little.

[Wim Boucquaert (wim-boucquaert)]

i tested it and all assets are sil numbers only, tested on the trunk

[Eric Casteleijn (thisfred)]

Ok, I'll have another look at this

[Eric Casteleijn (thisfred)]

fixed.

[Eric Casteleijn (thisfred)]

Bengt Giger reports:

> - assets without a proper suffix do not get the correct meta type after
> import, ie. PDFs are tagged as plain/html.

So it appears that the mime type isn't taken along correctly in the export, and only the file extension in the original zope id is used.