I can think of only a couple solutions then, none of which I have the ability to write the code for.

Option A:
If file is found, start search from beginning again, do this until not found

Option B:
Use a reverse order (we should never have a -8), Search for "filename" & "-8", if not found search for "filename" & "-7",...etc until say, "filename" &"-3" was found, then rename cell "-4"

Option C
Some crazy search that searches for the first 11 digits and through some wizardry defines a unique "-" number.