This request works for me:
[↓] [vPodans] $base64 = [io.file]::ReadAllText("C:\Users\vPodans\Desktop\req.txt")
[↓] [vPodans] $rawBytes = [convert]::FromBase64String($base64)
[↓] [vPodans] $pkcs7 = new-object System.Security.Cryptography.Pkcs.PKCS7SignedMessage(,$rawBytes)
[↓] [vPodans] $pkcs7
Version : 3
DigestAlgorithms : {sha1}
ContentType : System.Security.Cryptography.Oid
Content : {System.Security.Cryptography.X509Certificates.PublicKey}
Certificates :
RevocationLists :
Attributes : {}
SignerInfos : {System.Security.Cryptography.Pkcs.SignerInfo2}
RawData : {48, 130, 6, 132...}
[↓] [vPodans] $pkcs7.Content[0]
Version : 1
RequestType : PKCS10
Subject : CN=this is a test, CN=Another Common Name
SubjectDN : System.Security.Cryptography.X509Certificates.X500DistinguishedName
PublicKey : System.Security.Cryptography.X509Certificates.PublicKey
Attributes : {0, 0, 0}
Extensions : {System.Security.Cryptography.Oid, System.Security.Cryptography.Oid, System.Security.Cryptography.
Oid, System.Security.Cryptography.Oid...}
SignatureIsValid : True
SignatureAlgorithm : System.Security.Cryptography.Oid
RawData : {48, 130, 4, 70...}
[↓] [vPodans] $pkcs7.Content[0].Extensions | ?{$_.oid.value -eq "2.5.29.17"}
AlternativeNames Critical Oid RawData
---------------- -------- --- -------
{DNS Name=MoreDNS.test.iu.... False System.Security.Cryptograp... {48, 42, 130, 19...}
[↓] [vPodans] $pkcs7.Content[0].Extensions | ?{$_.oid.value -eq "2.5.29.17"} | fl *
AlternativeNames : {DNS Name=MoreDNS.test.iu.ed, DNS Name=onemore.test.iu.ed}
Critical : False
Oid : System.Security.Cryptography.Oid
RawData : {48, 42, 130, 19...}
[↓] [vPodans]
To make sure that you are doing it right: please post first 10 bytes from $rawBytes variable. Just type in PS console: $rawBytes[0..10]