Mime-Types lost in Firefox when file name contains spaces

When selecting a file from the fileshare that has spaces in its name, Firefox cannot determine the mime-type of the file. For example: "Test File.doc" registers as type "application/octet-stream" while "TestFile.doc" is recognized as a Microsoft Word document. (Please see attached screenshot.)

In my limited testing, this also happens with Excel files, but does not seem to affect PDF files.