new column from import file timestamp

Anonymous Please try replacing your code with below (You didn't remove the step I've mentioned to remove). Anyway, here it is. Hopefully it should work.

let
	Source = Folder.Files("E:\Docs\MyDocs\__One Drive IQT\OneDrive - INFO QUEST TECHNOLOGIES SA\_Leads_opportunities\Leads"),
	#"Filtered Hidden Files1" = Table.SelectRows(Source, each [Attributes]?[Hidden]? <> true),
	#"Invoke Custom Function1" = Table.AddColumn(#"Filtered Hidden Files1", "Transform File from _Leads_opportunities", each #"Transform File from _Leads_opportunities"([Content])),
	#"Renamed Columns1" = Table.RenameColumns(#"Invoke Custom Function1", {"Name", "Source.Name"}),
	#"Expanded Table Column1" = Table.ExpandTableColumn(#"Renamed Columns1", "Transform File from _Leads_opportunities", Table.ColumnNames(#"Transform File from _Leads_opportunities"(#"Sample File"))),
	#"Changed Type" = Table.TransformColumnTypes(#"Expanded Table Column1",{{"Source.Name", type text}, {"(Do Not Modify) Lead", type text}, {"(Do Not Modify) Row Checksum", type text}, {"(Do Not Modify) Modified On", type datetime}, {"Created On", type datetime}, {"Topic", type text}, {"Owner", type text}, {" Name", type text}, {"First Name", type text}, {"Middle Name", type any}, {"Last Name", type text}, {"Company Name", type text}, {"City", type text}, {"Description", type text}, {"Status Reason", type text}, {"Business Phone", type text}, {"Mobile Phone", type text}, {"Email", type text}, {"Lead Source", type text}, {"Job Title", type text}}),
	#"Inserted First Characters" = Table.AddColumn(#"Changed Type", "First Characters", each Text.Start([Source.Name], 4), type text),
	#"Inserted Text Range" = Table.AddColumn(#"Inserted First Characters", "Text Range", each Text.Middle([Source.Name], 5, 2), type text),
	#"Inserted Text Range1" = Table.AddColumn(#"Inserted Text Range", "Text Range.1", each Text.Middle([Source.Name], 8, 2), type text),
	#"Renamed Columns" = Table.RenameColumns(#"Inserted Text Range1",{{"First Characters", "Year"}, {"Text Range", "Month"}, {"Text Range.1", "Date"}}),
	#"Changed Type1" = Table.TransformColumnTypes(#"Renamed Columns",{{"Year", type number}, {"Month", type number}, {"Date", type number}}),
	#"Renamed Columns2" = Table.RenameColumns(#"Changed Type1",{{"Date", "Day"}})
in
	#"Renamed Columns2"