如何透视MySQL实体 - 属性 - 值模式

I need to design tables which stores all the metadata of files (i.e., file name, author, title, date created), and custom metadata (which has been added to files by users, e.g. CustUseBy, CustSendBy). The number of custom metadata fields cannot be set beforehand. Indeed, the only way of determining what and how many custom tags have been added on files is to examine what exists in the tables. I need to design tables which stores all the me