Increasing row number of Index

The only thing that Excel updates automatically is a relative cell reference. It never updates row and column numbers in INDEX, OFFSET, VLOOKUP, and similar reference functions. This makes perfect sense within the Excel concept.

Here is a simpler solution:

Formula:

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and copy and paste.

This may not work if you have both files open at the same time, and then insert rows on [Attachments_Email.xlsx]Sheet2. To immunize yourself against that situation use this:

Formula:

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The 2 is not the row number but the row in which the value is located and will not change
If your formula is in sy, row2 you can use the ROW() function which will adapt

Formula:

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Perhaps post a sheet ( see yellow banner) to describe what you are trying to achieve?

Glad to help, thanks for the rep!