When you drag and drop file with spaces in name, it correctly puts quotation mark at the beginning and the end of file name path. But when it is bracket ( or ) it doesn't. xfce-terminal should correctly do like mate-terminal, gnome-terminal, ... terminals do: all drag and drop files always quoted if it is needed or not. I thing it is better than checking for all exception and forget one
Unfortunately, you haven't specified the terminal version, but my guess is that it's 0.6.3, correct? I've tested current git version, and it's taking file names with brackets into single quotes. Actually, I'm thinking this is not related to the terminal itself but to the vte library it's using. Again, my guess is that you are comparing vte-based xfce4-terminal with vte3-based terminals - please confirm. If my guess is correct, the issue has gone with porting of xfce4-terminal to gtk3/vte3.
I tested both version(0.6.3 ubuntu 16.04 and 0.6.92 from https://launchpad.net/~xubuntu-dev/+archive/ubuntu/xfce4-gtk3 If I drag and drop file like a(.mp4 and path does not contain any spaces like /home/user/Downloads/a(.mp4 it doesnt quote file full path. Just now I tested with other problematic characters from https://en.wikipedia.org/wiki/File_name#Reserved_characters_and_words Note 1: While they are allowed in Unix file and folder names, most Unix shells require certain characters such as spaces, <, >, |, \, and sometimes :, (, ), &, ;, #, as well as wildcards such as ? and *, to be quoted or escaped: five\ and\ six\<seven (example of escaping) 'five and six<seven' or "five and six<seven" (examples of quoting) and xfce terminal fails to quote names too. Because this bug I was forced to find alternative; currently mate terminal 1.14.1 gtk3.
Thanks for the explanation. Indeed, quoting was done by the terminal which was relying on the g_shell_parse_argv() API; but it was not returning the expected result for file names containing brackets. So I changed the behavior to always quote file names and URIs. Fixed by https://git.xfce.org/apps/xfce4-terminal/commit/?id=16307a518080bef632592275c6696f47c1c18b89