cpanel 11.42.0 (build 23)
1. Compose new message
2. Attach file with name "file1_a.pdf" (attachment: 1_file1_attached.png)
3. Everything is ok.
4. Attach file with name "file2_ä.pdf" (attachment: 2_file2_attached.png)
5. File 1 seems to be ok, second file has been attached but without name
6. Attach file with name "file3_a.jpg" (attachment: 3_file3_attached.png)
7. First two attachments has disappear and only third attachment is visible
I also tested this with roundcube in same server (as there is Cpanel in use) and there everything was ok, so I believe this isn't related to server itself.