Moving Library from Mendeley to Zotero

I am trying to move about 900 files from Mendeley to Zotero. I saved the library as a .biblx (not sure if that is the correct ending) and imported it into Zotero. All the abstracts, file names and other information saved; however, only some of the files that were attached in Mendeley copied over. Does anyone know why? Also, is there a better way to do it to ensure all files transfers as well? Thank you!
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