I tried the two below variants and it found it each time. I hope this is what you are after:
B4X:
Dim MyFile As String=File.DirRootExternal & "/myfolder/" & "Base_Settings.tab"
If File.Exists("",MyFile) Then
Msgbox("I found it: " & MyFile,"")
Else
Msgbox("not found","")
End If
B4X:
Dim MyFile As String= "Base_Settings.tab"
If File.Exists(File.DirRootExternal & "/myfolder" ,MyFile) Then
Msgbox("I found it: " & MyFile,"")
Else
Msgbox("not found","")
End If