New io-datatype File (UAC-1 of #123)

[schlingling]

Introduces a new io-datatype File

Part of RFC0002 mobility-extension.

Implements following UACs of #123:

• [UAC-1] New io-datatype File is implemented.

[rhazn]

File type should be an enum of strings i think, can be only octed stream for now. Same with extension so that we avoid the issues of different values due to people using different capitalizations.

I'll approve this so you can merge this without re-requesting a review when you make the enum changes.

[schlingling]

File type should be an enum of strings i think, can be only octed stream for now. Same with extension so that we avoid the issues of different values due to people using different capitalizations.

I'll approve this so you can merge this without re-requesting a review when you make the enum changes.

Fyi, i will change the enums again in #126 to a version without Keys, this makes it easier for us when using Files.