Hey.. i believe this is a new bug. I always have a cover.jpg in every Album folder and i used to be able to transfer that one to my phone via the Foobar2000 mobile FTP server just fine.
But in 1.6.1 it seems to only allow music file types. E.g. .opus and .mp3 work. As soon as i try any other file the server just disconnects me.
[21:58:14] [L] MLST cover.jpg
[21:58:14] [L] 550 /Music on Internal Storage/1234/cover.jpg cannot be listed
[21:58:14] [L] PASV
[21:58:14] [L] 227 Entering Passive Mode (192,168,9,102,184,143).
[21:58:14] [L] Opening data connection IP: 192.168.9.102 PORT: 47247
[21:58:14] [L] STOR cover.jpg
[21:58:14] [L] 150 Opening BINARY mode data connection for cover.jpg
[21:58:14] [L] Connection Lost: 192.168.9.102 (Duration: 2 minutes 19 seconds)
[21:58:14] Transferred 0 Files (0 bytes) in 0,21 seconds (0 Kbps)
[21:58:14] [L] Transfer Failed: cover.jpg (256 KB of 397 KB) in 0,03 seconds (2,05 Mbps)
Workaround is to rename the file to something like cover.jpg.mp3, transfer it and then rename it again on the phone.
While that is annoying for adding single albums, it's impossible for larger transfers.