TRy this: In Event window click on 'Attach' in pop up window it says: 'Enter a webpage, or document location' remove the http:// Type: file:// then add the complete path to your local file i.e. file://C:\Users\Documents\test.pdf.

Thanks for your quick answer.

This is ok, but it is complicated and because you have to write in the entire file address, in an organization with limited skills (hospital in Tanzania) in data processing, this will not work.

Are there no way you can just click on the file you want to attach - as in an e-mail and like in Outlook... You really have to write it..?

Best regards, Sig

This question appears to have a long history:

https://bugzilla.mozilla.org/show_bug.cgi?id=168680

One user mentioned this in connection with Exchange calendar events, while another one indicates there are new options in the Event dialog once a Filelink account has been set up.

See also this report on the possible ineffectiveness of the suggested workaround.