I don’t see any such missing feature in DEVONthink.
All files imported into DEVONthink remain in their native filetypes.
Some filetypes can be directly displayed within a DEVONthink view, such as plain or rich text (using the OS X Cocoa text editor), HTML and WebArchive (using the OS X WebKit), and PDF (using the OS X PDFKit).
Some filetypes with a proprietary display requirement cannot be rendered directly within DEVONthink but are instead displayed using Apple’s Quick Look feature (the Quick Look plugin is supplied by the developer of the application that generated the document). Examples include MS Word, Excel, Powerpoint, Pages, Numbers, Keynote, Mellel, etc. The quality of Quick Look plugins varies widely. At a minimum, they should display the text content of the document. Some, such as that for Pages, provide a quite faithful rendering of the original document including images and layout. Others, such as that supplied by Microsoft for Word, do not provide a faithful rendering of the original document. (The next upgrade of OS X, Lion, will provide a much more faithful view of MS Office documents within OS X.)
The Finder maintains a database that lists the ‘parent’ application for data files, as well as a list of other applications capable of opening data files. (The ‘parent’ application can be defined in the Finder by selecting a file and opening its Info panel.)
Editing (and in many cases, printing) documents contained in a DEVONthink databases can be performed by opening the document in an external application. This can be done by selecting the document and Control-clicking on it, then choosing the contextual menu option, ‘Open With’. A more convenient way to do this is to add the ‘Open Externally’ option to the Toolbar (View > Customize Toolbar). Then select a document and click on the ‘Open Externally’ icon in the Toolbar; the document will open in its parent application as defined in the Finder.
The user may define different instances of the same filetype differently, in the Finder. For example, one image file might be defined as having GraphicConverter as the parent application, while another might be defined as having Photoshop as the parent application. Or all image files might be defined as having a specific parent application, but the ‘Open With’ command can allow one to choose another application for a specific need.
Note that a sortable column that lists the Kind of all documents can be added to a view window (View > Columns > Kind).