Trayitem files are used to hold Lot metadata that is used by the Sims 4. It is essentially a serialized Protobuf file, and uses VarInt to store numbers. An example dump of a trayitem file is as follows (incomplete):

Loading ./tray/0x00000002!0x023801bb9522020d.trayitem
Lot name: String (9): Rivendell
Lot Description: String (165): #fantasy #elven houses in a nice #green #valley with #waterfall, #slope, #stream, #cave, #bridge and different  charateristic rooms. NO CC, MOO used. By Artrui @ MTS
Creator UUID: VarInt: 2474641876
Creator Name: String (9): Blumenrui
Favourites: VarInt: 0
Downloads: VarInt: 0
Unknown: string(14) "01dfc608220a6f"
Lot Width: VarInt: 40
Lot Height: VarInt: 30
Lot Price Category (Low to High): VarInt: 6
Lot Value (Price Value): VarInt: 776843
Number of Bedrooms: VarInt: 3
Number of Bathrooms: VarInt: 4
Architecture Value: VarInt: 370593
Num Thumbnails: VarInt: 7
Front Side Thumbnail: VarInt: 0
Unknown2: VarInt: 4239733099
Unknown3: VarInt: 4

Full specification to follow.

Uint32 (0x00)
Uint32 (Bytes Until EOF)

Byte (0x08)

* Byte  (Changing this single byte = unreadable by the game.  Perhaps a checksum?)
Byte 
  (0xC7)
  (0xB4) - 
  (0x80) 
  (0x94) 

Byte
  (0x80)
  (0x94)
  (0xB8)
  (0xE4)
  (0x8C)
  (0x98)
  (0xD0)

(Above 2 bytes: Combinations seen)
  80 98
  80 94
  80 D0
  80 B8
  
  80 98 - Nothing on lot
  80 FC - Tags
  80 E4 - 1 trait
  80 D8 - 1 trait + Tags
  80 E0 - All 3 traits
  80 F4 - 1 trait 

Byte (Varies)

Byte x 4 
(92 AB C2 F5) 
(B9 B7 80 9C) 
(9C AB C2 9F)
(AC AB C2 B9)
(A6 AB C2 A9)

Byte x 1 (0F) (02) (08)

Byte x 2 (10 02)
Byte (10) (1A)
Byte x 4 (10 00 00 00)

Byte x 13 (0x00)

Byte (0x22)
Byte (Length)
Byte x Length (String) - Lot Name

Byte (0x2A)
Byte (Length)
Byte x Length (String) - Lot Description

Byte (0x30) - Creators UUID
Byte x 5 Seen: (F3 B9 A8 B1 08) (DD 80 E9 E4 8D)

Byte (0x3A)
Byte (Length)
Byte x Length (String) - Creators Name

Byte (0x40) - Favourites
Byte (0x00)?

Byte (0x48) - Downloads
Byte (0x00)?

Byte (0x52)
Byte x 3
Seen:
34 0A 20 
36 0A 22 
3E 0A 20
3E 0A 2A
46 0A 32 
4E 0A 3A 
51 0A 3A 

Byte x 4 (0x 08 C6 DF 01)

Byte (0x10)
Byte (Lot Width) Seen: 0x1E 0x28

Byte (0x18)
Byte (Lot Height) Seen: 0x14 0x1E

Byte (0x20) - Lot Price Category (1 - 6) aka Price Level
Byte (0x01)?

Byte (0x28) - Lot Value aka Price Value
Byte Seen: 0x32 (0xB4 01) (0x8B B5 2F)

Byte (0x30)
Byte Number of bedrooms

Byte (0x38)
Byte Number of bathrooms

Byte (0x40) Can be multiple bytes! - Architecture Value
Byte Seen (0x3C) (0xB4 01) (0x00)  (0x A1 CF 16)

Byte (0x48) Num Thumbnails
Byte Seen 0x03 0x04

Byte (0x50) Front side
Byte Seen 0x03

Byte (0x58) Venue type? + Groud floor index?
Byte x 5 (EB E2 D4 E5 0F)

Byte (0x60) 
Byte (0x02)

(Optional)
  Byte 0x72
  Byte x 9 (Trait requirements)  
  Seen:
    18 1E 33 02 00 00 00 00 00
    10 1E 33 02 00 00 00 00 00
    08 1E 33 02 00 00 00 00 00
    18 00 7B 02 00 00 00 00 00 1F 33 02 00 00 00 00 00
    18 1A 33 02 00 00 00 00 00 19 33 02 00 00 00 00 00
    

Byte x 6 (Building type?)
 78 00 20 00 2B 00) 


Byte (0x42) 
Byte 00 (Tags length)
(If tagLength > 0) Then read string of tags

Byte x 4 (0x50 10 58 00)

Byte (0x60)
Byte x 3 Seen: (0xC1 A8 09 ) (0xC0 EB 06) (0x AA B0 04) (0xBC B0 04)

Byte x 4  (CB or C9)
(CB 3E 9E 28) (CB 3E 9E 28) (C8 3E 9E 28)

(Optional?)
Byte 0x58
* Byte x 2 (These 2 vary on same lot saved twice)
  
Byte (AC)


Byte x 3
(A3 ED 01) (80 CA 04)

Bytes x 9
88 01 00 A0 01 00 AB 01 00