Following code with correct WHERE condition can be used to delete files attached to FeedItem after deleting FeedItem in Salesforce.

Sample Code:

Set < Id > setContentVerIds = new Set < Id >();
List < ContentDocument > listFiles = new List < ContentDocument >();

for ( 
    FeedItem FI : [ 
        SELECT Id, RelatedRecordId 
        FROM FeedItem 
    ] 
) { 
    
    //Add your required WHERE condition
    if ( 
        String.isNotBlank(
            FI.RelatedRecordId
        ) 
    ) {
        
        //RelatedRecordId is the Content Version Id
        setContentVerIds.add(
            FI.RelatedRecordId
        );
    }
    
}

for ( 
    ContentVersion CV : [ 
        SELECT Id, ContentDocumentId 
        FROM ContentVersion 
        WHERE Id IN: setContentVerIds 
    ] 
) {
    
    listFiles.add( 
        new ContentDocument( 
            Id = CV.ContentDocumentId 
        ) 
    );
    
}

if ( listFiles.size() > 0 ) {
    
    delete listFiles;
    
}