GMail does not let you upload attachments in Chrome and Firefox.
English: Attachment failed.
German: Anhang fehlgeschlagen.
Polish: Dodawanie załącznika nie powiodło się.
Google Toolbar uses Flash to talk to native Windows API when displaying the progress bar during attachment uploads. If you put your Internet Explorer to offline mode some parts of the Windows Explorer's API are unavailable. The progress bar can't be displayed and this will result in "Attachment failed" error message.
Go to Internet Explorer and uncheck the "Offline mode". Alternatively, remove Google Toolbar.