I've had this happen twice now so I'm curious whether or not others have had this happen, and more importantly, does anyone know what causes it or how to prevent it from happening.
We use JIRA in our office and shortly after the team upgraded to JIRA 5.x I was suddenly unable to attach files, but only PDFs and only in Firefox. JIRA would throw back a cryptic "Unknown server error (500)". After some searching around I discovered that it was because there was something wrong with my PDF mimetype in Firefox. Looking in Options | Application I discovered two entries for Adobe Acrobat Document, one as ("application/pdf") and the other as (application/pdf). A quick browse through my mimeTypes.rdf file I found the first entry as "application/pdf". Deleted this duplicate entry and it and fixed the problem.
Now I've had the same thing happen again but this time with WinZip files. It took me a minute to remember what caused it but sure enough duplicate entry with " surrounding it.