Combination If Function

It appears that in order to add any of the last four attachments, attachment 1 is necessary.
The drop down for cell A3 would only display Attachment 1
The drop down for cell A4 would include the last four attachments but only if attachment 1 is chosen in cell A3, otherwise nothing is displayed
The drop down for cell A5 will display the two appropriate attachments if attachment 2 is selected in cell A4, the one appropriate attachment if attachment 3 is selected in cell A4 and will not display any options otherwise.
This avoids the customer choosing attachments that will not go together.
Note that cells C3:C4 are populated using: =SUBSTITUTE(A3," ","_") and the results are hidden by choosing white as the font color.
Note that the dropdowns in cells A4:A5 are populated using =INDIRECT(C3) and =INDIRECT(C4) respectively.
Note that lists are placed in columns N:Q
Let us know if you have any questions.