Rework MIME Type handling

[jpfeuffer]

There are some points to consider:

1. Add an "any" or "*" MIME type, so some nodes can accept any type
2. Maybe add missing mime types on the fly. (With a single extension of the newly found one, and a name like "unknown-$extension"

[jpfeuffer]

Also
3) FileListPorts should probably have a MIMETypeSet to allow mixed lists.

[jpfeuffer]

And what if mimetypes overlap, e.g. when multiple plugins include an extension in one of their mime types. Probably also not handled here.

[jpfeuffer]

So I found the best possibility is just to use the extension as is, if no mimetype is found. Makes more sense than failing IMHO. A perfect match should always be allowed, only a fallback should be to compare possible Mimetype agreement.

[jpfeuffer]

Also, the CTDParser should be updated to read the "is_executable" tag for input files. Could help in putting some restrictions on the mine types even if no extension in the CTD is specified