Change default save file type

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Hi,

when I merge 3 files each around 100 mb Wireshark gives "the capture file appears to be damaged or corrupt pcap bigger than maximum of 262144" error. While select 3rd file it asks me to save first 2. If I directly save it default file type I got error. But if I select "modidfed pcap file" can able to save without any error.

So how can I change default file type "modified pcap file"? Is it possible?