I had an odd situation reported to me a week or so ago, and I finally figured out what was going on.
In this case, we have a list that supports attachments. The customer reported that they added six attachments to the list item (combination of .xlsx, .docx, and .pdf files). All of the files showed up in the attachment list, but once they saved the list item and opened it back up, the .pdf attachment was gone.
Of course, like all good IT issues, I tried the same thing and it kept the .pdf file I tried to attach… 🙂
When I spent some more time looking at her example, it occurred to me that the .pdf file name was rather long… like 131 characters worth. Given that the file name length maximum in SharePoint is 128 characters, that would be a problem.
So apparently what happens is that you can “attach” the file with the name that’s too long to the list item, but when you go to save it, it gets dropped. Unfortunately, you don’t get any indication that it did so… it’s just… gone.