Incorporating Group by Max in Table.SelectRows

Group and filter:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSjNU0lEC4WinzLzEospYpVgdqKgRVlFjDFEjrCYYYTXBCKcJJiiisQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [fileName = _t, fileIndex = _t, Content = _t]),
    ct = Table.TransformColumnTypes(Source,{{"fileName", type text}, {"fileIndex", Int64.Type}, {"Content", type text}}),

    #"Max fileIndex per filName" = Table.Group(ct,{"fileName"},{
        {"filtered", each Table.FromRecords({Table.Max(_,"fileIndex" )}),
            type table[fileName=text, fileIndex=Int64.Type, Content=text]}
        }),
    #"Expanded filtered" = Table.ExpandTableColumn(#"Max fileIndex per filName", "filtered", {"fileIndex", "Content"})
in
    #"Expanded filtered"

This might help. Check out my Approach #4 in particular:

https://community.powerbi.com/t5/Community-Blog/Select-Distinct-Rows-Ordered-by-Another-Column-Power-Query/ba-p/2141505

Here's a version with Table.Join instead of Table.NestedJoin:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSjNU0lEC4WinzLzEospYpVgdqKgRVlFjDFEjrCYYYTXBCKcJJiiisQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [fileName = _t, fileIndex = _t, Content = _t]),
    ct = Table.TransformColumnTypes(Source,{{"fileName", type text}, {"fileIndex", Int64.Type}, {"Content", type text}}),
    #"Grouped Rows" = Table.Group(ct, {"fileName"}, {{"fileIndex", each List.Max([fileIndex]), type nullable number}}),
    #"Merged Queries" = Table.Join(ct, {"fileName", "fileIndex"}, #"Grouped Rows", {"fileName", "fileIndex"}, JoinKind.Inner)
in
    #"Merged Queries"

In terms of performance, Table.Group() is better. All in all, try to use database to finish this task.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSjNU0lEC4WinzLzEospYpVgdqKgRVlFjDFEjrCYYYTXBCKcJJiiisQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [fileName = _t, fileIndex = _t, Content = _t]),
    ct = Table.TransformColumnTypes(Source,{{"fileName", type text}, {"fileIndex", Int64.Type}, {"Content", type text}}),
    Grouped = Table.Group(ct, "fileName", {"grp", each Table.Max(_, "fileIndex")}),
    #"Expanded grp" = Table.ExpandRecordColumn(Grouped, "grp", {"fileIndex", "Content"}, {"fileIndex", "Content"})
in
    #"Expanded grp"

-- for SQL Server
WITH cte_rn
     AS (SELECT *
                , Row_number ()
                    OVER (
                      partition BY filename
                      ORDER BY fileindex DESC) AS RN
         FROM   yearly_sales)
SELECT *
FROM   cte_rn
WHERE  rn = 1