<p>In an HTML form post what are valid characters for creating a multipart boundary?</p>

According to
RFC 2046
, section 5.1.1:
```
```
boundary := 0*69<bchars> bcharsnospace

 bchars := bcharsnospace / " "

 bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" /
                  "+" / "_" / "," / "-" / "." /
                  "/" / ":" / "=" / "?"
```
```
So it can be between 1 and 70 characters long, consisting of alphanumeric, and the punctuation you see in the list. Spaces are allowed except at the end.