A client was experiencing very strange behavior following an upgrade from MOSS 2007 to SharePoint 2010 Server. In a nutshell some documents such as Word (.doc and .docx) would open while others such as Excel (.xls and xlsx) and PDF documents would not open.
Note: By open I mean when clicking on an Excel document, stored within a document library, it would not open in Excel installed on Windows 7, Vista, XP, etc.
Furthermore, the file type icons that normally display next to familiar documents such as Excel were missing and a missing image red x was displayed.
The behavior of unrecognized File Types and missing document icons seemed to indicate an issue with the docIcon.xml configuration which is commonly located here: C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\XML.
However, after inspecting this file on each server I could find nothing apparently wrong. Finally, one last tedious inspection revealed the issue. Following the upgrade we had to reconfigure the Adobe PDF IFilter which meant adding the following key to the docIcon.xml file:
<Mapping Key="pdf" Value="pdficon_small.gif" />
After looking at this entry more closely I noticed that the double quotations marks looked italicized “. This meant the configuration contain extra characters and was most likely edit with an editor like word pad instead of note pad.
Tip: Never use Word Pad to edit configuration files because it will add additional characters and likely corrupt the configuration file.
I removed the unsupported double quotes and replaced them with unformatted double quotes. Viola, we could now open Excel and PDF Files again! Even better the cute little document type icons came back! :)
The reason the Excel Documents types became unavailable is because these document type mappings were located after the pdf file mapping within the docIcon.xml. Essential the invalid PDF entry broke all entries below it.
Hope this helps someone else!!!