We are trying to uplaod attachments to our confluence installation. When we upload a pdf file for example we get a preview of the pdf file in the editor but as soon as we are publishing the page, only an "unknown attachment" (in german its "Unbekannter Anhang", not sure what its called in english) symbol is visible to us.
As we start editing the page again we can hover the cursor above the unknown attachment and the files name is displayed correctly.
This happens not only with pdfs but with every other file, too.