diff --git a/packages/downgrader/src/v3.1-to-v3.0.ts b/packages/downgrader/src/v3.1-to-v3.0.ts
index c39b4b4..34cf95f 100644
--- a/packages/downgrader/src/v3.1-to-v3.0.ts
+++ b/packages/downgrader/src/v3.1-to-v3.0.ts
@@ -297,6 +297,15 @@ function convertType(out: Record<string, unknown>, type: unknown): boolean {
   return false
 }
 
+function contentFormat(schema: Record<string, unknown>): string | undefined {
+  if (schema.type !== undefined && !hasType(schema.type, 'string')) {
+    return undefined
+  }
+  return schema.contentEncoding === 'base64'
+    ? 'byte'
+    : schema.contentEncoding === undefined && typeof schema.contentMediaType === 'string' ? 'binary' : undefined
+}
+
 function finishSchema(out: Record<string, unknown>, schema: Record<string, unknown>, ctx: Context): unknown {
   if (typeof schema.$ref === 'string') {
     out.allOf = [convertSchemaRef(schema.$ref, ctx), ...allOfItems(out.allOf)]
@@ -328,10 +337,8 @@ function finishSchema(out: Record<string, unknown>, schema: Record<string, unkno
   if (Array.isArray(schema.examples) && schema.examples.length > 0 && !has(schema, 'example')) {
     out.example = clone(schema.examples[0])
   }
-  const format = schema.contentEncoding === 'base64'
-    ? 'byte'
-    : schema.contentEncoding === undefined && typeof schema.contentMediaType === 'string' ? 'binary' : undefined
-  if (format !== undefined && (schema.type === undefined || hasType(schema.type, 'string'))) {
+  const format = contentFormat(schema)
+  if (format !== undefined) {
     out.format ??= format
     if (schema.type === undefined) {
       out.type = 'string'
@@ -421,26 +428,34 @@ function formParts(schema: unknown, ctx: Context): Map<string, Map<unknown, stri
   return parts
 }
 
-function defaultsToOctetStream(schemas: Map<unknown, string>, ctx: Context, isItem = false): boolean {
+function hasBinaryFormat(node: Record<string, unknown>): boolean {
+  const format = node.format ?? contentFormat(node)
+  return format === 'binary' || format === 'byte'
+}
+
+function partContentType(schemas: Map<unknown, string>, ctx: Context, isItem = false): string | undefined {
   const bases = addSubschemas(schemas, ctx)
   const nodes = [...bases.keys()]
   if (!nodes.every(node => isRecord(node) || node === true)) {
-    return false
+    return undefined
   }
   const records = nodes.filter(isRecord)
   const types = records.flatMap(node => [node.type ?? []].flat())
   const kinds = new Set(types.filter(type => type !== 'null'))
   if (types.length === 0) {
-    return true
+    return 'application/octet-stream'
   }
   if (kinds.size !== 1) {
-    return false
+    return undefined
   }
   if (kinds.has('string')) {
-    return records.some(node => node.contentEncoding !== undefined)
+    if (records.some(node => node.contentEncoding !== undefined)) {
+      return 'application/octet-stream'
+    }
+    return records.some(hasBinaryFormat) ? 'text/plain' : undefined
   }
   if (isItem || !kinds.has('array')) {
-    return false
+    return undefined
   }
   const items = new Map<unknown, string>()
   for (const [node, base] of bases) {
@@ -448,7 +463,7 @@ function defaultsToOctetStream(schemas: Map<unknown, string>, ctx: Context, isIt
       place(items, item, base, ctx)
     }
   }
-  return items.size === 0 || defaultsToOctetStream(items, ctx, true)
+  return partContentType(items, ctx, true)
 }
 
 function finishFormMediaType(out: Record<string, unknown>, mediaType: Record<string, unknown>, ctx: Context): unknown {
@@ -458,12 +473,12 @@ function finishFormMediaType(out: Record<string, unknown>, mediaType: Record<str
   }
   for (const [name, schemas] of formParts(mediaType.schema, ctx)) {
     const entry = child(encoding, name) ?? {}
-    if (
-      isRecord(entry)
-      && !CONTENT_TYPE_OVERRIDES.some(key => Object.hasOwn(entry, key))
-      && defaultsToOctetStream(schemas, ctx)
-    ) {
-      setOwn(encoding, name, { ...entry, contentType: 'application/octet-stream' })
+    if (!isRecord(entry) || CONTENT_TYPE_OVERRIDES.some(key => Object.hasOwn(entry, key))) {
+      continue
+    }
+    const contentType = partContentType(schemas, ctx)
+    if (contentType !== undefined) {
+      setOwn(encoding, name, { ...entry, contentType })
       out.encoding = encoding
     }
   }
diff --git a/packages/downgrader/tests/v3.1-to-v3.0/spec/form-bodies.test.ts b/packages/downgrader/tests/v3.1-to-v3.0/spec/form-bodies.test.ts
index bd6e26b..f0decce 100644
--- a/packages/downgrader/tests/v3.1-to-v3.0/spec/form-bodies.test.ts
+++ b/packages/downgrader/tests/v3.1-to-v3.0/spec/form-bodies.test.ts
@@ -12,11 +12,18 @@
 //   other strings → text/plain, `object` → application/json,
 //   `array` → the default of its `items`, and nothing for a missing `type`
 //
-// The schema conversion keeps most of these aligned, but not an untyped
-// part (`{}` has no 3.0 default) or a `contentEncoding` that `format: byte`
-// cannot express (such as base64url, which becomes a plain string and so
-// text/plain). For those parts the 3.1 default, application/octet-stream,
-// is written into the Encoding Object so the wire format stays the same.
+// The schema conversion keeps most of these aligned, but not:
+// - an untyped part (`{}` has no 3.0 default) or a `contentEncoding` that
+//   `format: byte` cannot express (such as base64url, which becomes a plain
+//   string and so text/plain). 3.1 sends these as application/octet-stream.
+// - a string without `contentEncoding` that has `format: binary` or `byte`,
+//   or gains `format: binary` from `contentMediaType`. 3.1 sends these as
+//   text/plain, 3.0 as application/octet-stream.
+// For those parts the 3.1 default is written into the Encoding Object so
+// the wire format stays the same. `contentMediaType` does not change the
+// 3.1 default, and a contradicting one is ignored
+// (https://spec.openapis.org/oas/v3.1.2.html#working-with-binary-data),
+// so it does not become the part's `contentType`.
 //
 // An Encoding Object that sets `style`, `explode`, or `allowReserved`
 // switches the part to RFC6570-style serialization, where `contentType`
@@ -34,7 +41,14 @@ import { convertComponent, convertSpec } from './helpers'
 
 const octetStream = { contentType: 'application/octet-stream' }
 
-const schemas = { Form: { allOf: [{ properties: { a: {} } }], properties: { b: {} } }, Pet: { type: 'object' }, Raw: {} }
+const textPlain = { contentType: 'text/plain' }
+
+const schemas = {
+  Form: { allOf: [{ properties: { a: {} } }], properties: { b: {} } },
+  Pet: { type: 'object' },
+  Png: { contentMediaType: 'image/png', type: 'string' },
+  Raw: {},
+}
 
 function convertForm(mediaType: unknown, type = 'multipart/form-data'): unknown {
   return dig(convertComponent('requestBodies', { content: { [type]: mediaType } }, { schemas }), 'content', type)
@@ -117,11 +131,29 @@ describe('parts that need the 3.1 default written out', () => {
     expect(Object.getPrototypeOf(encoding)).toBe(Object.prototype)
     expect(Object.getOwnPropertyDescriptor(encoding, '__proto__')?.value).toEqual(octetStream)
   })
+
+  it.each([
+    ['a string with contentMediaType', { contentMediaType: 'image/png', type: 'string' }],
+    ['a binary string', { format: 'binary', type: 'string' }],
+    ['a byte string', { format: 'byte', type: 'string' }],
+    ['a nullable string with contentMediaType', { contentMediaType: 'text/csv', type: ['string', 'null'] }],
+    ['an array of strings with contentMediaType', { items: { contentMediaType: 'image/png', type: 'string' }, type: 'array' }],
+    ['a string with a binary format found through allOf', { allOf: [{ format: 'binary' }], type: 'string' }],
+    ['string anyOf branches, one of them binary', { anyOf: [{ format: 'byte', type: 'string' }, { type: 'string' }] }],
+    ['a reference to a string with contentMediaType', { $ref: '#/components/schemas/Png' }],
+  ])('sets contentType: text/plain on %s', (_name, part) => {
+    expect(convertForm({ schema: { properties: { part } } })).toEqual({
+      encoding: { part: textPlain },
+      schema: { properties: { part: expect.anything() } },
+    })
+  })
 })
 
 describe('parts whose 3.0 default already matches', () => {
   it.each([
     ['a string', { format: 'uuid', type: 'string' }],
+    ['a string whose format wins over contentMediaType', { contentMediaType: 'text/csv', format: 'uuid', type: 'string' }],
+    ['a binary format on a non-string type', { format: 'binary', type: 'integer' }],
     ['an object', { type: 'object' }],
     ['a type found through allOf', { allOf: [{ $ref: '#/components/schemas/Pet' }] }],
     ['a null type', { type: 'null' }],
@@ -170,6 +202,19 @@ describe('existing Encoding Objects', () => {
     expect(convertForm({ encoding, schema }, 'application/x-www-form-urlencoded')).toEqual({ encoding, schema })
   })
 
+  it('treats a text/plain default like an application/octet-stream one', () => {
+    const headers = { 'X-Id': { schema: { type: 'string' } } }
+    const binary = { format: 'binary', type: 'string' }
+    const schema = { properties: { explicit: binary, headed: binary, styled: binary } }
+    expect(convertForm({
+      encoding: { explicit: { contentType: 'image/png' }, headed: { headers }, styled: { style: 'form' } },
+      schema,
+    })).toEqual({
+      encoding: { explicit: { contentType: 'image/png' }, headed: { ...textPlain, headers }, styled: textPlain },
+      schema,
+    })
+  })
+
   it('leaves an Encoding Object shared with another part unchanged', () => {
     const entry = { headers: { 'X-Id': { schema: { type: 'string' } } } }
     const result = dig(convertComponent('requestBodies', {