`Generate` uses wrong file type for `.storyboard` and `.xib` files

The following is generated from `xcsync generate` for storyboard files:
```
"16D316C00000000000000000": {
"path": "Main.storyboard",
"isa": "PBXFileReference",
"name": "Main.storyboard",
"lastKnownFileType": "text.plist.xml",
"sourceTree": ""
},
```

This is what is created using Xcode:
```
B905DB222CC20BFF0070EC34 /* Base */ = {
isa = PBXFileReference;
lastKnownFileType = file.storyboard;
name = Base;
path = Base.lproj/Main.storyboard;
sourceTree = ""; };

```