Confirming the "source file will be discarded" warning coerced every
numeric column of the preview with parseInt, which truncated the
fraction: 0.901045951946194 became 0, and 1234.56 became 1234.
parseFloat converts the same values without dropping precision, and a
cell the parser could not read as a number is left as text so the
editor's numeric validation flags it instead of showing NaN.
Adds an Excel spec case that reads the first data row before and after
the switch to edit mode and asserts the decimal value survives.
dc