Deleting other files during maintenance plan
As per MS here


  The Maintenance Cleanup task removes files related to maintenance
  plans, including database backup files and reports created by
  maintenance plans.
  
  By using the Maintenance Cleanup task, a package can remove the backup
  files or maintenance plan reports on the specified server. The
  Maintenance Cleanup task includes an option to remove a specific file
  or remove a group of files in a folder. Optionally you can specify the
  extension of the files to delete.


I have not deleted any other extension files via maintenance plans and seems from above that a non sql file may not be deleted via Maint'ce plans:

What we use is a VB script and schedule it via SQL agent job:

Select type as :- Type ActiveX scipt when you create the step for below code in SQL agent job:

On Error Resume Next   
    Dim fso, folder, files, sFolder, sFolderTarget     
    Set fso = CreateObject("Scripting.FileSystemObject")   

    'location of the  files 
    sFolder = "B:\FolderA\ABC\"

    Set folder = fso.GetFolder(sFolder)   
    Set files = folder.Files     

    'used for writing to textfile - generate report on files deleted 
    Const ForAppending = 8 

    'you need to create a folder named "scripts" for ease of file management &amp;  
    'a file inside it named "LOG.txt" for delete activity logging 
    Set objFile = fso.OpenTextFile(sFolder &amp; "\Report\LOG.txt", ForAppending) 

    objFile.Write "================================================================" &amp; VBCRLF &amp; VBCRLF 
    objFile.Write "                     FILE REPORT                " &amp; VBCRLF 
    objFile.Write "                     DATE:  " &amp;    FormatDateTime(Now(),1)   &amp; "" &amp; VBCRLF 
    objFile.Write "                     TIME:  " &amp;    FormatDateTime(Now(),3)   &amp; "" &amp; VBCRLF &amp; VBCRLF 
    objFile.Write "================================================================" &amp; VBCRLF  

    'iterate thru each of the files in the respective folder 
    For Each itemFiles In files  
       'retrieve complete path of file for the DeleteFile method and to extract  
            'file extension using the GetExtensionName method 
       a=sFolder &amp; itemFiles.Name 

       'retrieve file extension  
       b = fso.GetExtensionName(a) 
           'check if the file extension is abc(whatever is the extension of you're file
           If uCase(b)="abc" Then 

               'check if the files are older than 1days 
               If DateDiff("d",itemFiles.DateCreated,Now()) &gt;= 1 Then 

                   'Delete any old files to cleanup folder 
                   fso.DeleteFile a  
                   objFile.WriteLine "FILE DELETED: " &amp; a 
               End If 
           End If 
    Next   

    objFile.WriteLine "================================================================" &amp; VBCRLF &amp; VBCRLF

    objFile.Close 

    Set objFile = Nothing 
    Set fso = Nothing 
    Set folder = Nothing 
    Set files = Nothing


Note* Above scripts work such that you need to create folder Report within you're base folder location and within that place a file LOG.txt which captures or logs the details of files deleted by the process. You can use this additional feature which we use to track, else modify the script as per needs!