If you're using a text editor such as NotePad++ then then the recommended solution is to change the transfer mode to BINARY and transfer all files in BINARY mode.
This will preserve the file formatting and integrity of the file.
The problem you're noticing can be caused by using ASCII mode to transfer the files or in this case (auto) mode. If you transferred the file in BINARY mode then the line endings would be identical on the server and the local copy.
These days many FTP servers do not allow ASCII transfers by default (.conf setting) because this mode requires more overhead and system resources and while these FTP servers may appear to allow ASCII mode the transfer will in fact be in BINARY.
And some FTP servers can accidentally mangle text documents when sent in ASCII mode. For example if the FTP server is running on a LINUX/UNIX OS and the text file is CR/LF formatted then attempting to transfer the file in ASCII mode may result in CR/CR/LF. (which is most likely the cause of your specific problem)
When setting the transfer mode to ASCII or BINARY this operation is a server side feature, the FTP server is responsible for performing the conversion. On the client side the data received or sent the same way regardless of the transfer mode.
Some FTP clients have supplemented the ASCII/Binary conversion by always using BINARY mode and then by performing the text conversion within the client software, however FlashFXP does not do this.
How to set the default transfer mode to Binary (image)
Start FlashFXP
Then from the main menu select
Options >
Preferences
From the
Preferences dialog select
Transfer
Change the
Transfer Mode to
Binary (image)
Click the
OK button
Now when you start FlashFXP it will default to use Binary (image) for the Transfer Mode.
(it does not however change the transfer mode for the current instance of FlashFXP)