After migrating to JIRA 6.3, we lost all attachments which JIRA Sequencial is greater or equal 10000. it seems (going to the directory where attachments are saved) that JIRA made a file from a directory where the are supposed to be.
For instance, Issue 27349:
ls in the directory shows:
/opt/jira.../data/attachments/CCF/CCF-27349/34567 <- HERE is a file but should be a directory holding the attachments.
/opt/jira.../data/attachments/CCF/CCF-27349/thumbs
Version of Jira: v6.3.15#6346-sha1:dbc023d