Years later... there may be a way.
Open regedit as administrator and head to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Portable Devices\FormatMap\
Backup this "FormatMap".
Next, we'll try to fool Windows...
Basically, you'll have to copy the content of "Format" and "ContentType" keys from a format to another.
Try to use these source formats :
.gif, .bmp for images (jpeg, png...) :
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Portable Devices\FormatMap\.gif]
"Format"="{38070000-AE6C-4804-98BA-C57B46965FE7}"
"ContentType"="{EF2107D5-A52A-4243-A26B-62D4176D7603}"
.wma for audio (mp3,mp4, aac, flac...) :
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Portable Devices\FormatMap\.wma]
"Format"="{B9010000-AE6C-4804-98BA-C57B46965FE7}"
"ContentType"="{4AD2C85E-5E2D-45E5-8864-4F229E3C6CF0}"
.wmv for video (avi mp4...) :
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Portable Devices\FormatMap\.wmv]
"Format"="{B9810000-AE6C-4804-98BA-C57B46965FE7}"
"ContentType"="{9261B03C-3D78-4519-85E3-02C5E1F50BB9}"
.doc, .xls, .ppt for documents or other files :
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Portable Devices\FormatMap\.doc]
"Format"="{BA830000-AE6C-4804-98BA-C57B46965FE7}"
"ContentType"="{680ADF52-950A-4041-9B41-65E393648155}"
e.g., the original key for flac is
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Portable Devices\FormatMap\.flac]
"Format"="{B9060000-AE6C-4804-98BA-C57B46965FE7}"
"ContentType"="{4AD2C85E-5E2D-45E5-8864-4F229E3C6CF0}"
So we'll just change it to WMA's ID:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Portable Devices\FormatMap\.flac]
"Format"="{B9010000-AE6C-4804-98BA-C57B46965FE7}"
"ContentType"="{4AD2C85E-5E2D-45E5-8864-4F229E3C6CF0}"
We could also add new formats, like .m4a (audio mp4) :
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Portable Devices\FormatMap\.m4a]
"Format"="{B9010000-AE6C-4804-98BA-C57B46965FE7}"
"ContentType"="{4AD2C85E-5E2D-45E5-8864-4F229E3C6CF0}"
However, it may have unwanted effects on any app using theses keys... Maybe WMP ?