If one migrates JIRA from one host server to another and the OS users use inconsistent system character encoding, in some case JIRA won't be able to access the attachments.
When user tries to access an imported attachment the application server returns 404.
HTTP Status 404 - Could not find specified attachment: /opt/lib/jira/attachments/TST/TST-133/10259_Präsentation1.ppt (No such file or directory)
type Status report
message Could not find specified attachment: /opt/jira/attachments/TST/TST-133/10259_Präsentation1.ppt (No such file or directory)
description The requested resource (Could not find specified attachment: /opt/jira/attachments/TST/TST-133/10259_Präsentation1.ppt (No such file or directory)) is not available.
In the same time the imported files are visible if the content of the attachment directory (/opt/jira/attachments/TST/TST-133/) is listed.
If that is the case, you can either change back the system encoding to the original one (locale) or alternatively migrate your attachments to the new one with help of convmv.
In order to permanently address this issue, we could replace the attachments' file names with numeric ID or store attachments in database JRA-806.