Add a command `/save_file`

[Bardin08]

This command contains 2 steps:

1. Receiving file, receiving session ID (GUID from 1-st to 8-th symbol). Ex: aea02346
2. Checking the received file by:
   • Size
   • File name (regex pattern or several)
   • File extension (Ex: *.pdf)

All the specified constraints can be set while creating a file receiving session