Hey guys,
I have been struck here from the last two days please help me in this,
>the first screen shot is mydetailpage in that page, slected file field which is url type. 

>"open VF page" button is my custo button, on click of that button I'm opening my VF Page.
VF Page Logic:
<apex:page standardController="MyCustom_2__c" extensions="openmyvfpage" >
   <apex:form >
       <apex:pageBlock >
           <apex:pageBlockSection columns="1">
            
            <apex:pageBlockTable value="{!lst}" var="a" id="accTable">
                    <apex:column headerValue="Select" >
                        <input type="radio" name="<strong>selectRadio</strong>" id= "radio">
                        <apex:param assignTo="{!selectedAttachementId}" name="accname" value=" {!a.id}"/>
                        </input>
                    </apex:column>
                    <apex:column value="{!a.ContentDocument.LatestPublishedVersion.Title}"/>
               </apex:pageBlockTable>
      
           <apex:commandButton value="Save" action="{!test}" rendered="{!IF(warning==null,true,false)}"  />
            </apex:pageBlockSection>
           <apex:pageBlockSection rendered="{!IF(warning==null,false,true)}">
             {!warning}
           </apex:pageBlockSection>
       </apex:pageBlock>
   </apex:form>
</apex:page>
> the second screen shot is my vf page preview

> from vf page i need to select one radion button, on click of save it should redirect to my detail page with updated value of "selected file" field
>My Controller logic:
public class openmyvfpage {
    
    public List<ContentDocumentLink> lst {set;get;}
    public id  currentURL {set;get;}
    public string warning {set;get;}
    public String selectedAttachementId {get;set;}
    
    public openmyvfpage(ApexPages.StandardController controller) {
        currentURL = apexpages.currentpage().getparameters().get('id');
        System.debug('>>>'+currentURL);
        List<MyCustom_2__c> parentid=[SELECT MyCustom_1__c FROM MyCustom_2__c WHERE Id=:currentURL LIMIT 1];
        System.debug('>>>>'+parentid);
        if(parentid[0].MyCustom_1__c!=null){
        lst=[SELECT Id, ContentDocument.LatestPublishedVersion.Title
                        FROM ContentDocumentLink 
                        WHERE LinkedEntityId =:parentid.get(0).MyCustom_1__c];
            if(lst.size()<0){
              warning='For this Record there are no attachemnts found';  
            }
        }else{
        warning='For this Record there are no attachemnts found';
        }
    }
        public PageReference test(){
            system.debug('>>'+selectedAttachementId);
            system.debug('>>>'+currentURL);
                 MyCustom_2__c my =new MyCustom_2__c();
                my.id=currentURL;
                my.Selected_File__c=selectedAttachementId;
                update my;
                PageReference p= new PageReference('/'+currentURL);
                p.setRedirect(true);
                return p;
            }
}
The issue I'm facing here is I'm unable to capture the selected radio button record id.