FormData: Pass raw bytes buffer as file input and allow substitution

[zekroTJA]

The [FormData] request block currently has tow issues:

1) Substitution

Variable substitution does currently not work in the field values.

Example:

[FormData]
someFile1 = @{{.file1Name}}:image/png
someFile2 = "{{.file2Contents}}"

2) Raw bytes buffer

It should be possible to pass a raw byte buffer as parameter to the value of a FormData entry. Because it must not be treated as a string, it would be required to do some modifications how raw values are parsed. Otherweise, there could be a new identifier for raw inputs like in the following example.

[FormData]
rawData = $rawData