git reset filename.txt

Will remove a file named filename.txt from the current index(Staging area), the "about to be committed" area, without changing anything else.

git reset <file>

Basically, It removes <file> from the staging(index) area, but it leaves the working directory unchanged. This un-stages a file without overwriting any changes.

git reset filename.txt

will remove a file named filename.txt from the current index  (also called the “staging area”, which is where changes “about to be committed” are saved), without changing anything else (the working directory is not overwritten).

git reset filename.txt

Will remove a file named filename.txt from the current index, the "about to be committed" area, without changing anything else.

git reset filename.txt

Will remove a file named filename.txt from the current index(Staging area), the "about to be committed" area, without changing anything else.

git reset <file>

Basically, It removes <file> from the staging(index) area, but it leaves the working directory unchanged. This un-stages a file without overwriting any changes.