Add option to replace special chars in temp filename

[nisrael]

It would be really great if you would include this in your version and create a new relase. We have problems reading the created temp files in our DMS system because of characters like #, § and so on in the filename, which defaults to the subject of the email. So this option, if enabled, replaces all non a-z,A-Z,0-9 characters with the underline character '_'.

[dhartmann84]

@tonyfederer Do you already know when the patch will be applied? We have the same problem with us with the #. Many thanks.