After moving the file from tmp to the desired directory, the functions, for some reason, return false or error?

Question

PHP

On WordPress, uploading files via a feedback form (self-written, by me)
Files are uploaded to tmp (tmp_name), then functions transfer files to /uploads /
.
displayed in admin/media files. And even the standard move_uploaded_file(). All files were transferred to the /uploads/ folder, all appeared and are read, but the functions return an error.
A passing question. If you send files by mail. Do they need to be transferred from tmp or can they be sent from tmp right away? Rather, the files on the server are not needed, but only in the letter in the place with the message.