Hello I am using a third party app on my mac called classic FTP which allows you to transfer files to a website. Although the program is not fully accessible for voice over it can be used especially by advanced users of mac although I would not recommended it for beginners.
A few tips then for you.
First insure any file you want to transfer using the program is in your documents folder of the apple mac. This is because the voice over reader doesn't tell you names of folders in the FTP program but the default it looks for is documents. This does also mean that yes you can have any format of file in the documents folder.
Second thing is insure particularly if you host a website that in the FTP program itself you have the correct user names port configurations password and any other details the FTP program requires.
Third thing is this sighted assistance is a strong recommendation even for advanced users especially when using this program for the first few times once you get the hang of the interface however most people should be able to get on without much sighted assistance.
Fourth if transferring files isn't working correctly apart from contacting the developers I would also highly recommend contacting your ISP or website hosting service. I have found my Australian ISP extremely useful and helpful in relation to all issues I've had so far.
Ok I am not going to give the direct link to this program here as you will only need it if you are a host or admin of a website google will bring it up for you the program does have a free trial then yes you do have to pay a fee I paid about $50 Australian but this is allowing for conversion from Australian currency to US money so prices may be cheaper in the US or other countries.
Finally, don't bother with the preferences in the classic FTP program without sighted assistance this also should mean you turn VO off as it doesn't read the preferences pains anyhow