Formulario para cargar imagenes

<script runat="client">
                var loadFile = function(e, fichero) {
                    if (fichero == "file"){
                        document.getElementById("imgUpload").style.display= "block";
                        document.getElementById("imgUpload").src = URL.createObjectURL(e.target.files[0]);
                    }
<script> 
    function readURL(input) {
        if (input.files && input.files[0]) {
            var reader = new FileReader();
 
            reader.onload = function (e) {
                $('#imgUpload').attr('src', e.target.result);
            }
 
            reader.readAsDataURL(input.files[0]);
        }
    }
 
    $("#imgInp").change(function(){
        readURL(this);
    }); 
</script>
                }
                function getBase64(e, fichero) {
                    var CIF = document.getElementById("CIF").value;
                    var reader = new FileReader;
                    reader.readAsDataURL(e), reader.onload = function() {
                        //prepare data to pass to processing page 
                         var fullFileName = document.getElementById("imgInp").files[0].name;
                         var asset = fullFileName.split(".")[1];
                         if (asset == "png" || asset == "jpg" || asset == "jpeg") {
                             var assetName = fullFileName.split(".")[1];
                             } else {
                               var assetName = "png"
                             }
                         var base64enc = reader.result.split(";base64,")[1],
                          fileName = fullFileName.split(".")[0]

                        fetch("https://cloud.xxx.com/xxxx", {
                                      method: "POST",
                                      headers: {
                                        "Content-Type": "application/json"
                                      },
                                      body: JSON.stringify({
                                        base64enc: base64enc,
                                        fileName: fileName,
                                        assetName: assetName,
                                        CIF: CIF
                                      })
                                      }).then(function(e) {
                                    }).catch(function(e) {
                                    })
                                }, reader.onerror = function(e) {
                                    }
                                }


                document.getElementById("button").addEventListener("click", function() {
                        if(document.getElementById("imgInp").files.length>0) {
                        var e = document.getElementById("imgInp").files;
                        e.length > 0 && getBase64(e[0],"imgInp");
                        }else{
                        }
                    });

</script>
Auxiliar para meter la imagen
<script runat="server">
    Platform.Load("Core","1.1.1");
    try {
        //fetch posted data
        var jsonData = Platform.Request.GetPostData();
        var obj = Platform.Function.ParseJSON(jsonData);
        //prepare data for API call
        var base64enc = obj.base64enc;
        var fileName = obj.fileName;
        var assetName = obj.assetName;
        var CIF = obj.CIF;
        var today = new Date();
        today.setTime(today.getTime() + (7*60*60*1000));
        var finalFileName= CIF + "" + today;
        //match asset type with uploaded file (https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-apis.meta/mc-apis/base-asset-types.htm)
        var assetTypes = { ai: 16, psd: 17, pdd: 18, eps: 19, gif: 20, jpe: 21, jpeg: 22, jpg: 23, jp2: 24, jpx: 25, pict: 26, pct: 27, png: 28, tif: 29, tiff: 30, tga: 31, bmp: 32, wmf: 33, vsd: 34, pnm: 35, pgm: 36, pbm: 37, ppm: 38, svg: 39, "3fr": 40, ari: 41, arw: 42, bay: 43, cap: 44, crw: 45, cr2: 46, dcr: 47, dcs: 48, dng: 49, drf: 50, eip: 51, erf: 52, fff: 53, iiq: 54, k25: 55, kdc: 56, mef: 57, mos: 58, mrw: 59, nef: 60, nrw: 61, orf: 62, pef: 63, ptx: 64, pxn: 65, raf: 66, raw: 67, rw2: 68, rwl: 69, rwz: 70, srf: 71, sr2: 72, srw: 73, x3f: 74, "3gp": 75, "3gpp": 76, "3g2": 77, "3gp2": 78, asf: 79, avi: 80, m2ts: 81, mts: 82, dif: 83, dv: 84, mkv: 85, mpg: 86, f4v: 87, flv: 88, mjpg: 89, mjpeg: 90, mxf: 91, mpeg: 92, mp4: 93, m4v: 94, mp4v: 95, mov: 96, swf: 97, wmv: 98, rm: 99, ogv: 100, indd: 101, indt: 102, incx: 103, wwcx: 104, doc: 105, docx: 106, dot: 107, dotx: 108, mdb: 109, mpp: 110, ics: 111, xls: 112, xlsx: 113, xlk: 114, xlsm: 115, xlt: 116, xltm: 117, csv: 118, tsv: 119, tab: 120, pps: 121, ppsx: 122, ppt: 123, pptx: 124, pot: 125, thmx: 126, pdf: 127, ps: 128, qxd: 129, rtf: 130, sxc: 131, sxi: 132, sxw: 133, odt: 134, ods: 135, ots: 136, odp: 137, otp: 138, epub: 139, dvi: 140, key: 141, keynote: 142, pez: 143, aac: 144, m4a: 145, au: 146, aif: 147, aiff: 148, aifc: 149, mp3: 150, wav: 151, wma: 152, midi: 153, oga: 154, ogg: 155, ra: 156, vox: 157, voc: 158, "7z": 159, arj: 160, bz2: 161, cab: 162, gz: 163, gzip: 164, iso: 165, lha: 166, sit: 167, tgz: 168, jar: 169, rar: 170, tar: 171, zip: 172, gpg: 173, htm: 174, html: 175, xhtml: 176, xht: 177, css: 178, less: 179, sass: 180, js: 181, json: 182, atom: 183, rss: 184, xml: 185, xsl: 186, xslt: 187, md: 188, markdown: 189, as: 190, fla: 191, eml: 192, text: 193, txt: 194, freeformblock: 195, textblock: 196, htmlblock: 197, textplusimageblock: 198, imageblock: 199, abtestblock: 200, dynamicblock: 201, stylingblock: 202, einsteincontentblock: 203, webpage: 205, webtemplate: 206, templatebasedemail: 207, htmlemail: 208, textonlyemail: 209, socialshareblock: 210, socialfollowblock: 211, buttonblock: 212, layoutblock: 213, defaulttemplate: 214, smartcaptureblock: 215, smartcaptureformfieldblock: 216, smartcapturesubmitoptionsblock: 217, slotpropertiesblock: 218, externalcontentblock: 219, codesnippetblock: 220, rssfeedblock: 221, formstylingblock: 222, referenceblock: 223, imagecarouselblock: 224, customblock: 225, liveimageblock: 226, livesettingblock: 227, contentmap: 228, jsonmessage: 230 };
        var assetTypeID = assetTypes[assetName];
        //authenticate to get access token
        var authEndpoint = 'https://xxxxx.marketingcloudapis.com/'; //add authentication endpoint
        var payload = {
            client_id: "xxxxxx", //pass Client ID
            client_secret: "xxxxxx", //pass Client Secret
            grant_type: "client_credentials"
        };
        var url = authEndpoint + '/v2/token'
        var contentType = 'application/json'
        var accessTokenRequest = HTTP.Post(url, contentType, Stringify(payload));
        if (accessTokenRequest.StatusCode == 200) {
            var tokenResponse = Platform.Function.ParseJSON(accessTokenRequest.Response[0]);
            var accessToken = tokenResponse.access_token
            var rest_instance_url = tokenResponse.rest_instance_url
        }
        //make api call to create asset   
        if (base64enc != null) {
            var headerNames = ["Authorization"];
            var headerValues = ["Bearer " + accessToken];
            var jsonBody = {
                "name": finalFileName,
                "assetType": {
                    "name": assetName,
                    "id": assetTypeID
                },
               "category": { "id": 392124 } //lo sacas del inspeccionador, seleccionando en la carpeta y viendo el data-id del <li>
               ,
               "file": base64enc
            };
            var requestUrl = rest_instance_url + "asset/v1/content/assets"
            var createAsset = HTTP.Post(requestUrl, contentType, Stringify(jsonBody), headerNames, headerValues);
        }

    } catch (error) {
        Write("<br>error: " + Stringify(error));
    }
</script>