My preliminary guess would be that, as a security precaution, Flash wouldn't let you interact with files on an end user's hard drive. I seem to recall that there's an option to set a certain Flash file as "Trusted", but this requires a relative lot of user intervention.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 6.0 x64; en-US; rv:18.104.22.168) Gecko/2008071912 Minefield/3.0.1 (mmoy CE K8Q-X06)
Laptop: HP Compaq nx6325 - Turion 64 X2 @ 2GHz, 2GB DDR2, 100GB HD, ATI Radeon X300, 15" LCD, Seven Pro
Handheld: Palm Treo 650 - Intel PXA270 @ 312MHz, 10MB RAM, 32MB flash, 2.7" LCD, Palm OS 5.4