Side-Band HTTP Payload concern

Hello experts! I am having an issue sending a HTTP Payload to another server. My goal is to intercept a File upload and send it to another Virtual Server (which has a ASM Policy applied). The main app base64 encodes the file upload and transmits the file within a URL Form encoded post. I have an iRule which intercepts the file decodes the base64 into a variable. The issue is when I attempt to append the needed Multi Part headers and footers to the capture decoded file, the resulting file becomes unreadable.

Code
when RULE_INIT {
    set static::debug 1
    set static::content [ifile get ]
}
when HTTP_REQUEST { 
    set VirusDetected 0
set output 0
    if { ([HTTP::uri] contains "") && ( [HTTP::method] eq "POST") } {
    set clen [HTTP::header Content-Length]
if {$static::debug} {log local0. "Request Length $clen" }
    HTTP::collect $clen
    } 
}
when HTTP_REQUEST_DATA {
    set payload [HTTP::payload]
    if {[URI::query "?$payload" ] eq "SaveAttachment" } {
     set IsAdaptEnabled 1
     set parametervalue [URI::query "?$payload" ]
     set parametervalue [URI::decode $parametervalue]
     set parametervalue [URI::decode $parametervalue]
     set contenttype1 [expr { [string first "data:" $parametervalue] + 5  } ]
     set contenttype2 [expr { [string first ";" $parametervalue $contenttype1] - 1 } ]
     set contenttypesnip [string range $parametervalue $contenttype1 $contenttype2];
     set filename1 [expr { [string first "FileName" $parametervalue] + 9  } ]
     set filename2 [expr { [string first "FileSize" $parametervalue $filename1] - 2 } ]
     set filenamesnip [string range $parametervalue $filename1 $filename2];
     set len1 [expr { [string first "base64," $parametervalue] + 7  } ]
     set len2 [expr { [string first "`" $parametervalue $len1] - 1 } ]
     set filesnip [string range $parametervalue $len1 $len2];
     if {[catch {b64decode $filesnip}  decodedfilesfile ] == 0 and $decodedfilesfile ne ""} { 
         base64 decoding succeeded 
     } else { 
         base64 decoding failed 
     } 
     set decodedfilesfilelength [string length $decodedfilesfile]
     if {$decodedfilesfilelength <= 20971520} {
            set firstboundary  "----WebKitFormBoundaryPMvLP4u7CcWWqOv7 \r\n"
            set lastboundary " \r\n\r\n----WebKitFormBoundaryPMvLP4u7CcWWqOv7\r\n"
            set contentdisposition "Content-Disposition: form-data; name=$filenamesnip; filename=$filenamesnip; size=$decodedfilesfilelength\r\nContent-Type: $contenttypesnip\r\n\r\n"
            set payload $firstboundary$contentdisposition$decodedfilesfile$lastboundary
            set s1 [call /Common/HSSR::http_req -state hstate -virt ASM_Internal_Virtual_Server -uri "http:///VirusCheck-for-filename/$filenamesnip" -method POST -body $payload -type "multipart/form-data ----WebKitFormBoundaryPMvLP4u7CcWWqOv7" -debug 1 -wait 60]
            if {$static::debug} {log local0. "ASM Virtual Server Responce $s1"}
            if {($s1 == 403) || ($s1 == 555)} {
                set VirusDetected 1
                HTTP::payload replace 0 $clen null
                HTTP::release
            } else {
                HTTP::release
            }
            call /Common/HSSR::http_close hstate
        } else {
            HTTP::respond 200 content "$static::content" noserver
        }
    }
}