What does the prefix of the raw transaction encoding of an exported Electrum transaction file (.txn) mean?

I have exported a testnet transaction into a .txn file:

{     "hex": "45505446ff0002000000000101c6be24d86ab1ae5b54e7e7905b...",     "complete": false,     "final": false } 

However, as explained in this article, a raw transaction string starts with version number and:

The version number is four bytes long and is expressed as a hexadecimal value in little endian format.

There are two version types. Version 01 indicates that there is no relative time lock. Version 02 indicates that there may be a relative time lock.

So this hex is not a qualified raw transaction encoding, due to the prefix

45505446ff00 

The version number comes after this prefix:

02000000 

So I was wondering, what is the function of this prefix that Electrum adds to the raw transaction string? And is it always the same length? Because I need to automatically splice it off in order to get a valid raw transaction string.