Overview:
While sending attachments in Email through Email to Case functionality, customer is getting 101 Too many SOQL Governor Limit Exception and thus Case is not getting created at their end.
Root Cause:
Triggers on ContentDocumentLink and ContentDocument is not bulkified at SFDC`s end, so whenever multiple files are uploaded together, all the Triggers written on ContentDocument or ContentDocumentLink object are fired one by one for each file and this is causing the 101 Too many SOQL Governor Limit Exception.
Solution:
This is working as designed by SFDC. The workaround is to either save the files as attachments instead of files or to reduce the number of SOQL queries in the customer`s triggers.
Related Documentation:
SFDC Help Documentation: ContentDocumentLink causes non-bulk execution of triggers on ContentVersion
SFDC IdeaExchange: Bulkify ContentDocumentLink trigger in case of email-to-case