Assuming that you are looking for sum(Degradation_Time) by file and then percentage, not count. See my answer below:
eventtype="Start" AND (NOT host=E* AND NOT host=I*)
| eval Degradation_Time=coalesce('Durée de la dégradation','Degradation Time','Tiempo de degradación','Beeinträchtigungszeit')
| eval File_Name=coalesce(Nom_du_fichier,File_Name,Dateiname,Nombre_de_archivo)
| stats sum(Degradation_Time) as Degradation_Time_By_File by File_Name
| appendcols
[search eventtype="Start" AND (NOT host=E* AND NOT host=I*)
| eval Degradation_Time=coalesce('Durée de la dégradation','Degradation Time','Tiempo de degradación','Beeinträchtigungszeit')
| stats sum(Degradation_Time) as Total_Degradation_Time]
| filldown Total_Degradation_Time
| eval Percent_Degradation_Time_By_File =(Degradation_Time_By_File*100)/Total_Degradation_Time
Hope it works
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