Document Thumbnails not showing

Nov 9, 2012
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Your Mac's Specs
MBP Late 2014, 2.2 GHz Intel Core i7, 16 GB 1600 MHz DDR3, Yosemite 10.10.3
Hey guys. I've been doing some data migrating recently. Most of my document files for various apps now live on my external drive. This has resulted in Finder not recognising and displaying the file thumbnail for many of the documents attributed to various apps.
For example all of my MS Office documents (word, docs, spreadsheets, etc) now just have a white documented thumbnail instead of a green Excel or blue Word thumbnail. Projects files from music applications like Reason are also showing just a white thumbnail.
Its as is Finder no longer knows which application to open these documents with but when right click & go to "Open With" the correct application is always set to the default.

This also means that I cant properly use Filter to filter documents by application type.

Can anyone advise?

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