General database questions

Hello. As someone might have seen already from my post in the PHP forum, I'm kind of new to dynamic web design. I have some questions regarding MySQL, mostly general ones:

I am in the process of using an IPB forum as the user database for my site. I would like to know how this process is done, and if there are any tips to follow so that I can take advantage of the groups system, as well as add new fields by myself.

Also, I want to know what is the best way to store files in a website, for backup, restoration, and download processes. I've read a little about the blob data type in a MySQL database, and it appears to be generic binary storage. Can I use this to store the files directly in the database? is it a good practice? will it work with files as big as 50Mb?

Thanks in advance.

ive never stored files in my database, so i dont know about the second part.
I often organize my files by name and link in my database, but i keep them on an ftp. Not in the database.

As for the user and groups system, my website devfile.com has my scripts in paralell to the forum scripts(phpbb) i didnt want to try and edit the phpbb talbles so i made my own and all the data that is shared i enter into both, then take from either one depending on if your using the main website or the forums.
this is probably bad practice, but i didnt want to mess up phpbb and i figured it would only be a minor setback.

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It's not a good idea to put large files in a database. I don't even put small ones in.