ParamSweep: Cannot change input files

I want to do a parameter sweep where I change the Domain which is loaded from image files.

I can select the Domain/Image/path to do a ParamSweep. However, in the ParamSweep view, whenever I try to make a sequence like this

"/path/to/file1.tif";"/path/to/file2.tif";"/path/to/file3.tif"

the GUI will open a file browser instead of a line editor. See image below. Therefore, I cannot make a sequence to do a ParamSweep.