The moodle file system doesn't consist of filename.ext files but rather meta-data from the mdl_files table - contenthash column - whose value is not only the location of the file but the filename in the sea of files in moodledata/filedir/
So your file explorer won't work unless you can get it to search for files named like: 30f8f3b48dc932162bd2d673f7fdb01387cea58c
When you migrated to another server, did you also migrate moodledata/filedir/ contents?
There is one chance to recover one course IF ... that's IF ... you made a course backup and it was saved to moodledata/filedir/.
Those backup file names end with .mbz and the meta-data in mdl_files table will help you locate them.
Do you have the database from the old server?
Via DB client either CLI or PHPMyAdmin, query to find backup files:
select id,contenthash,filename,filesize from mdl_files where filename like '%.mbz';
Example (* below) of what might be returned:
* | 3650885 | 30f8f3b48dc932162bd2d673f7fdb01387cea58c | backup-moodle2-course-9-pre-calculus-20180601-1332-nu.mbz | 429238267 |
In * above, 30f8f3b48dc932162bd2d673f7fdb01387cea58c is filename in moodledata/filedir/ and it is located in moodledata/filedir/30/f8/
It's humanly recognizable name is:
backup-moodle2-course-9-pre-calculus-20180601-1332-nu.mbz
IF you still have the moodledata/filedir/ directory on old server and have command line (ssh) access, one could check the information above with a file -b command from moodledata/filedir/ to see if the mime-type indicates the file to be a gzip which .mbz files are.
As per example and located in moodledata/filedir/
find ./ -name 30f8f3b48dc932162bd2d673f7fdb01387cea58c
Would show:
./30/f8/30f8f3b48dc932162bd2d673f7fdb01387cea58c
Commaned file -b on file above will return:
"gzip compressed data, from Unix"
It is a .mbz and you could copy that file out to some other location and rename it ... somebackup.mbz + use to restore to your other moodle once that file is transferred to the new server.
'SoS', Ken