XNB_Version4_spec.txt

byte
CHAR[3]				"XNB"
Byte				target platform, w - windows, m - windows phone 7, x - xbox 460
Byte				version (4 - XNA 3.1, 5 - XNA 4.0)
Byte				Flag bits (0x1 HiDef, 0x80 compressed data - from typereader count onwards)
Uint32				File size (total, including header [85? bytes?))
Uint32				(Optional) Decompressed file size
7BitEncodedInt			Type reader count
Struct[Type reader Count]	Typereader array
 - String			The class-path to the type reader (with textures its "Microsoft.XNA.Content.Texture2DReader")
 - Int32			Version (0 usually/always)
7BitEncodedInt			Shared Resource Count (0 usually/always)
Object				Object Data
 - 7BitEncodedInt		TypeId (Gets you Typereader[TypeId - 1])
 - ObjectData			Data in the format the TypeReader[TypeId-1] expects

 Texture2DReader
 - Int32			SurfaceFormat enum, 0x01 - Color (ARGB32)
 - Int32			Width
 - Int32			Height
 - Int32			Number of mipmap levels
 - Int32			Number of bytes of image data
 ARGB32_data
 - Byte[width*height*4] For each pixel, 4 bytes for ARGB channels, one channel in each byte