Intercept files meta DnD upload breaks on filesize check

[troywilliams]

When the intercept files setting is enabled and set to meta the drag and drop breaks due to filesize check in the YUI javascript.

maxbytes = -1 may be due to a call to get_user_max_upload_file_size() which can return a -1 value which indicates filesize limits can be ignored for this user.

Moodle v3.7.x

Troy