Here are the correct Microsoft Office MIME types for HTTP Content Streaming:

Extension MIME Type
.doc      application/msword
.dot      application/msword

.docx     application/vnd.openxmlformats-officedocument.wordprocessingml.document
.dotx     application/vnd.openxmlformats-officedocument.wordprocessingml.template
.docm     application/vnd.ms-word.document.macroEnabled.12
.dotm     application/vnd.ms-word.template.macroEnabled.12

.xls      application/vnd.ms-excel
.xlt      application/vnd.ms-excel
.xla      application/vnd.ms-excel

.xlsx     application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
.xltx     application/vnd.openxmlformats-officedocument.spreadsheetml.template
.xlsm     application/vnd.ms-excel.sheet.macroEnabled.12
.xltm     application/vnd.ms-excel.template.macroEnabled.12
.xlam     application/vnd.ms-excel.addin.macroEnabled.12
.xlsb     application/vnd.ms-excel.sheet.binary.macroEnabled.12

.ppt      application/vnd.ms-powerpoint
.pot      application/vnd.ms-powerpoint
.pps      application/vnd.ms-powerpoint
.ppa      application/vnd.ms-powerpoint

.pptx     application/vnd.openxmlformats-officedocument.presentationml.presentation
.potx     application/vnd.openxmlformats-officedocument.presentationml.template
.ppsx     application/vnd.openxmlformats-officedocument.presentationml.slideshow
.ppam     application/vnd.ms-powerpoint.addin.macroEnabled.12
.pptm     application/vnd.ms-powerpoint.presentation.macroEnabled.12
.potm     application/vnd.ms-powerpoint.template.macroEnabled.12
.ppsm     application/vnd.ms-powerpoint.slideshow.macroEnabled.12

.mdb      application/vnd.ms-access

For further details check out this TechNet article and this blog post.