Attachments from Classic are not searchable in Lightning Experience

Lightning Experience Users can still view legacy attachments
 

• Lightning Experience Users can still view legacy attachments on records though they will not find them when performing a search from within Lightning.

Organizations that have Users who use Lightning Experience and Salesforce Classic 

• It is highly recommended to enable the feature 'Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments' for organizations who have both Lightning Experience and Salesforce Classic Users. 
• Enabling this feature ensures that any attachments added to records will be uploaded as Files so that Lightning Users are able to search for them within Lightning Experience.
• Attachments uploaded after enabling this feature will be returned in Lightning search results under Files.

Switch to Salesforce Classic to find attachments added before enabling the feature 'Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments'
 

• In order to locate attachments that have been added prior to enabling this feature, Lightning Users will need to temporarily switch to Salesforce Classic to perform a search there in order to locate legacy attachments.

Attachments from Email-to-Case
 

• To understand how this affect attachments from cases created by Email-to-Case, please review Attachments in email-to-case not getting attached to the created case record.

See Also:
Files are uploaded as Salesforce Files and not as Attachments
Before enabling the enhanced note-taking tool Notes and using Files