Change when form parts go to files

[randy909]

Rather than copy all form values to temp files when they have a content
type defined, only copy when they specify a filename or when the
content type is "application/octet-stream". Otherwise leave them as
"normal" key-value pairs also using the charset to construct the strings.

[LordFokas]

It's been a long time since that file stopped existing. You should have pulled the current master before making the changes, now you have a conflict on your hands.

Also you should change the partIsFile() method to protected, a user might want to override that logic some time later (for example, to only dump to disk files bigger than a certain size or to dump some relevant non-file data into files automatically)