Please login via terminal and do a "ls -al" command on the extensions folder to look what is inside. Verify the plugin file has the right user account (fmserver) and the right group (fmsadmin). If needed, use command line tools chown and chgrp to change the user and group.
Of course the byte size of the file should match to the copy on your own computer, so the upload completed. Next check file permissions. The file must be readable by the server process, so read permission is at least needed.
Finally, check the file type with the "file" command to make sure you got the Linux version and not the Windows one. So you need ELF 64-bit for x86-64 for Linux and not the PE32 DLL for Windows.
If everything works, the file should be able to load after turning on plugins in Admin Console. After a fresh install you may need to restart server once, so FileMaker scans the extensions folder for plugin files.
Please do not hesitate to contact us in case of questions.