diff --git a/src/api/posts.ts b/src/api/posts.ts
index 212c861..9830649 100644
--- a/src/api/posts.ts
+++ b/src/api/posts.ts
@@ -11,11 +11,12 @@ export interface Attachment {
 export async function uploadAttachment(
   authToken: string,
   base64data: string,
+  mediaType: string,
 ): Promise<Attachment> {
   const url = `${BACKEND_URL}/api/v1/media`
   const response = await http(url, {
     method: "POST",
-    json: { file: base64data },
+    json: { file: base64data, media_type: mediaType },
     authToken,
   })
   const data = await response.json()
diff --git a/src/components/PostEditor.vue b/src/components/PostEditor.vue
index b5fa8af..98b3a08 100644
--- a/src/components/PostEditor.vue
+++ b/src/components/PostEditor.vue
@@ -230,10 +230,11 @@ async function onAttachmentUpload(event: Event) {
     return
   }
   const imageDataUrl = await fileToDataUrl(files[0])
-  const imageBase64 = dataUrlToBase64(imageDataUrl)
+  const imageData = dataUrlToBase64(imageDataUrl)
   const attachment = await uploadAttachment(
     ensureAuthToken(),
-    imageBase64,
+    imageData.data,
+    imageData.mediaType,
   )
   attachments.push(attachment)
 }
diff --git a/src/utils/upload.ts b/src/utils/upload.ts
index 8ee60a5..0f760b7 100644
--- a/src/utils/upload.ts
+++ b/src/utils/upload.ts
@@ -1,3 +1,5 @@
+const DATA_URL_REGEXP = /^data:(\w+\/[-+.\w]+);base64,([a-zA-Z0-9+/]+={0,2})$/
+
 export async function fileToDataUrl(file: File): Promise<string> {
   return new Promise((resolve, reject) => {
     const reader = new FileReader()
@@ -10,6 +12,10 @@ export async function fileToDataUrl(file: File): Promise<string> {
   })
 }
 
-export function dataUrlToBase64(dataUrl: string): string {
-  return dataUrl.replace(/^data:.+;base64,/, "")
+export function dataUrlToBase64(dataUrl: string): { mediaType: string, data: string } {
+  const match = dataUrl.trim().match(DATA_URL_REGEXP)
+  if (!match) {
+    throw new Error(`invalid data URL ${dataUrl}`)
+  }
+  return { mediaType: match[1], data: match[2] }
 }
diff --git a/src/views/ProfileForm.vue b/src/views/ProfileForm.vue
index b0789fc..8a0734f 100644
--- a/src/views/ProfileForm.vue
+++ b/src/views/ProfileForm.vue
@@ -163,7 +163,8 @@ async function onFilePicked(fieldName: "avatar" | "header", event: Event) {
   }
   const imageDataUrl = await fileToDataUrl(files[0])
   images[fieldName] = imageDataUrl
-  form[fieldName] = dataUrlToBase64(imageDataUrl)
+  const imageData = dataUrlToBase64(imageDataUrl)
+  form[fieldName] = imageData.data
 }
 
 function isValidExtraField(index: number): boolean {
diff --git a/tests/unit/upload.spec.ts b/tests/unit/upload.spec.ts
new file mode 100644
index 0000000..35b773f
--- /dev/null
+++ b/tests/unit/upload.spec.ts
@@ -0,0 +1,10 @@
+import { expect } from "chai"
+import { dataUrlToBase64 } from "@/utils/upload"
+
+describe("Data URL", () => {
+  it("Should parse data URL", () => {
+    const dataUrl = "data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIyNCIgaGVpZ2h0PSIyNCIgdmlld0JveD0iMCAwIDI0IDI0IiBmaWxsPSJub25lIiBzdHJva2U9ImN1cnJlbnRDb2xvciIgc3Ryb2tlLXdpZHRoPSIxLjUiIHN0cm9rZS1saW5lY2FwPSJyb3VuZCIgc3Ryb2tlLWxpbmVqb2luPSJyb3VuZCIgY2xhc3M9ImZlYXRoZXIgZmVhdGhlci1zZWFyY2giPjxjaXJjbGUgY3g9IjExIiBjeT0iMTEiIHI9IjgiPjwvY2lyY2xlPjxsaW5lIHgxPSIyMSIgeTE9IjIxIiB4Mj0iMTYuNjUiIHkyPSIxNi42NSI+PC9saW5lPjwvc3ZnPg=="
+    const { data, mediaType } = dataUrlToBase64(dataUrl)
+    expect(mediaType).to.equal("image/svg+xml")
+  })
+})