A change was introduced for RESTEASY-1779 to encode non-US-ASCII filenames per RFC 5987, but the fix was incorrect/incomplete. The filename is encoded using URLEncoder, which specifically replaces a space character (" ") with a plus sign ("+"). However, RFC 5987 3.2.1 specifies that any character not included in their attr-char set (defined in the same section should have their byte representation percent encoded per RFC 3986 section 2.1.

Spaces are not included in the attr-char set defined in RFC 5987 3.2.1, so spaces should be percent-encoded per RFC 3986 as "%20":

https://tools.ietf.org/html/rfc3986#section-2.1

This bug results in some clients being unable to read a filename. For example, prior to this change, "Test file.pdf" would be show in the content-disposition header as:

filename="Test file.pdf".

After the change, it shows incorrectly as:

filename*=utf-8''Test+File.pdf

The correct encoding should be:

filename*=utf-8''Test%20file.pdf