Relax mxc regexp

The spec doesn't say which characters are permitted in the opaque ID,
but in real life we at least need a hyphen. For lack of a better
reference, I copied gomuks.

src/matrix/api.js

@@ -474,7 +474,7 @@ async function ping() {
 async function getMedia(mxc, init = {}) {
 	init = {...init}
 
-	const mediaParts = mxc?.match(/^mxc:\/\/([^/]+)\/(\w+)$/)
+	const mediaParts = mxc?.match(/^mxc:\/\/([^/]+)\/([a-zA-Z0-9_-]+)$/)
 	assert(mediaParts)
 
 	let route = "download"

src/matrix/utils.js

@@ -250,7 +250,7 @@ function getPublicUrlForMxc(mxc) {
  */
 function makeMxcPublic(mxc) {
 	assert(hasher, "xxhash is not ready yet")
-	const mediaParts = mxc?.match(/^mxc:\/\/([^/]+)\/(\w+)$/)
+	const mediaParts = mxc?.match(/^mxc:\/\/([^/]+)\/([a-zA-Z0-9_-]+)$/)
 	if (!mediaParts) return undefined
 
 	const serverAndMediaID = `${mediaParts[1]}/${mediaParts[2]}`