Our customers report an issue from time to time that sometimes Midifiles are not visible especially on Yamaha devices after downloading them.
At the Yamaha forum we have found a thread reporting that the amount of characters seems to be limited to 46 (plus file extension '.mid' thus 50 in total).
That's not much so we have to handle this fact.
We therefore removed the 'full details' (artist + title) from our default file name selection, as we received too many inquiries regarding this issue.
Customer who chose this setting as their default are auomatically set to 'song name' file naming.
When renaming please be sure that you do not remove the '.mid' if it is visible (on most computers file extensions are hidden by default).