Undelete trigger event in apex trigger in Salesforce

Before Undelete

The record is available, but the field isDeleted set to True. the before undelete trigger wouldn’t be able to see the records(values) that it needs. due to this reason, there is no before undeleting operation available.

After undelete

The after undelete trigger event only works with recovered records—that is, records that were deleted and then recovered from the Recycle Bin through the undelete DML statement. These are also called undeleted records.

Ex:
If you delete an Account, an Opportunity may also be deleted. When you recover the Account from the Recycle Bin, the Opportunity is also recovered.
If there is an after undelete trigger event associated with both the Account and the Opportunity, only the Account after undelete trigger event executes.

The after undelete trigger event only fires for the following objects:

• Account
• Asset
• Campaign
• Case
• Contact
• ContentDocument
• Contract
• Custom objects
• Event
• Lead
• Opportunity
• Product
• Solution
• Task

Salesforce ContentDocumentLink Trigger

Topics Covered in this trigger
1.   Changing Filename based on Parent Record.
2.   Avoiding Duplicate Files insertion.
3.   Checking File Name Size.

Note: Salesforce not supporting to show custom error message while uploading files. maybe in future salesforce resolve this issue.
For more information about issue check below link
Custom Error message Not Showing When File Uploading

Example:
In this Example, I am checking only account object you can extend this functionality as per your requirement.

ContentDocumentLinkTrigger

trigger ContentDocumentLinkTrigger on ContentDocumentLink (after insert) {
    if(trigger.isAfter) {
        if(trigger.isInsert) {
            ContentDocumentLinkTriggerHandler.onAfterInsert(trigger.new);
        }
    }
}

ContentDocumentLinkTriggerHandler

public inherited sharing class ContentDocumentLinkTriggerHandler {
    
    public static void onAfterInsert(list<ContentDocumentLink> lstCntLinks) {
        String strObjPrefix;
        Set<Id> setCntDocIds = new set<Id>();
        set<Id> setAccIds = new set<Id>();
        map<Id, Account> mapAccs;
        
        for(ContentDocumentLink clIterator : lstCntLinks) {
            strObjPrefix = String.valueOf(clIterator.LinkedEntityId).substring(0, 3);
            if(strObjPrefix == Account.sObjectType.getDescribe().getKeyPrefix()) {
                setCntDocIds.add(clIterator.ContentDocumentId);
                setAccIds.add(clIterator.LinkedEntityId);
            }
        }
        
        if(!setCntDocIds.isEmpty()) {
            if(!setAccIds.isEmpty()) {
                mapAccs = new map<Id, Account>([SELECT Id, Name FROM Account WHERE Id IN :setAccIds]);
            }
        }
        
        // Get content document object for current set of files
        map<Id, ContentDocument> mapContentDocuments = new map<Id, ContentDocument>([SELECT Id, Title, FileExtension FROM ContentDocument WHERE Id IN :setCntDocIds]);
        
        // Retrieve all the existing attachments associated with the parent records
        map<Id, set<String>> mapParentIdFilenames = new map<Id, set<String>>();
        for (ContentDocumentLink cdlIterator : [SELECT Id, ContentDocumentId, LinkedEntityId, ContentDocument.Title, ContentDocument.FileExtension FROM ContentDocumentLink WHERE LinkedEntityId IN :setAccIds AND ContentDocumentId NOT IN :setCntDocIds]) {
            if (!mapParentIdFilenames.containsKey(cdlIterator.LinkedEntityId)) {
                mapParentIdFilenames.put(cdlIterator.LinkedEntityId, new set<String>());
            }
            mapParentIdFilenames.get(cdlIterator.LinkedEntityId).add(cdlIterator.ContentDocument.Title + (String.isBlank(cdlIterator.ContentDocument.FileExtension) ? '' : '.' + cdlIterator.ContentDocument.FileExtension));
        }
        
        Account objAcc;
        list<ContentDocument> lstCntDocsToUpdate = new list<ContentDocument>();
        
        for(ContentDocumentLink cdlIterator : lstCntLinks) {
            ContentDocument objCntDoc = mapContentDocuments.get(cdlIterator.ContentDocumentId);
            String strFilename = '';
            if(mapAccs.containsKey(cdlIterator.LinkedEntityId)) {
                objAcc = mapAccs.get(cdlIterator.LinkedEntityId);
                strFilename = objAcc.Name + '_Account_' + objCntDoc.Title + '.' + objCntDoc.FileExtension;
            }
            
            if(!String.isBlank(strFilename)) {
                set<String> setExistingFilenames = mapParentIdFilenames.get(cdlIterator.LinkedEntityId);
                if(objCntDoc.Title.length() > 40) {
                    cdlIterator.addError('Filename to long.');
                }
                else if (setExistingFilenames != null && setExistingFilenames.contains(strFilename + (String.isBlank(objCntDoc.FileExtension) ? '' : '.' + objCntDoc.FileExtension))) {
                    cdlIterator.addError('Attaching duplicate file, please choose different file.');
                }
                else {
                    objCntDoc.Title = strFilename;
                    lstCntDocsToUpdate.add(objCntDoc);
                }
            } 
        }
        if(!lstCntDocsToUpdate.isEmpty()) {
            update lstCntDocsToUpdate;
        }
        
    }
}

Result

Resources
ContentDocumentLink
ContentVersion
ContentDocument