diff --git a/src/ooxml/java/org/apache/poi/openxml4j/opc/PackagingURIHelper.java b/src/ooxml/java/org/apache/poi/openxml4j/opc/PackagingURIHelper.java
index d74e0a13ef..c4169d9dbf 100644
--- a/src/ooxml/java/org/apache/poi/openxml4j/opc/PackagingURIHelper.java
+++ b/src/ooxml/java/org/apache/poi/openxml4j/opc/PackagingURIHelper.java
@@ -484,7 +484,7 @@ public final class PackagingURIHelper {
 			throws InvalidFormatException {
 		URI partNameURI;
 		try {
-			partNameURI = new URI(partName);
+			partNameURI = new URI(resolvePartName(partName));
 		} catch (URISyntaxException e) {
 			throw new InvalidFormatException(e.getMessage());
 		}
@@ -646,4 +646,29 @@ public final class PackagingURIHelper {
 		}
 		return retPartName;
 	}
+
+    /**
+     *  If  part name is not a valid URI, it is resolved as follows:
+     * <p>
+     * 1. Percent-encode each open bracket ([) and close bracket (]).</li>
+     * 2. Percent-encode each percent (%) character that is not followed by a hexadecimal notation of an octet value.</li>
+     * 3. Un-percent-encode each percent-encoded unreserved character.
+     * 4. Un-percent-encode each forward slash (/) and back slash (\).
+     * 5. Convert all back slashes to forward slashes.
+     * 6. If present in a segment containing non-dot (?.?) characters, remove trailing dot (?.?) characters from each segment.
+     * 7. Replace each occurrence of multiple consecutive forward slashes (/) with a single forward slash.
+     * 8. If a single trailing forward slash (/) is present, remove that trailing forward slash.
+     * 9. Remove complete segments that consist of three or more dots.
+     * 10. Resolve the relative reference against the base URI of the part holding the Unicode string, as it is defined
+     * in ?5.2 of RFC 3986. The path component of the resulting absolute URI is the part name.
+     *</p>
+     *
+     * @param partName the name to resolve
+     * @return  the resolved part name that should be OK to construct a URI
+     *
+     * TODO YK: for now this method does only (5). Finish the rest.
+     */
+    public static String resolvePartName(String partName){
+        return partName.replace('\\', '/');
+    }
 }
diff --git a/src/ooxml/java/org/apache/poi/openxml4j/opc/ZipPackage.java b/src/ooxml/java/org/apache/poi/openxml4j/opc/ZipPackage.java
index 2e211a64d1..114c75c06e 100644
--- a/src/ooxml/java/org/apache/poi/openxml4j/opc/ZipPackage.java
+++ b/src/ooxml/java/org/apache/poi/openxml4j/opc/ZipPackage.java
@@ -130,7 +130,7 @@ public final class ZipPackage extends Package {
 		Enumeration<? extends ZipEntry> entries = this.zipArchive.getEntries();
 		while (entries.hasMoreElements()) {
 			ZipEntry entry = entries.nextElement();
-			if (entry.getName().equals(
+			if (entry.getName().equalsIgnoreCase(
 					ContentTypeManager.CONTENT_TYPES_PART_NAME)) {
 				try {
 					this.contentTypeManager = new ZipContentTypeManager(
@@ -208,7 +208,7 @@ public final class ZipPackage extends Package {
 		try {
 			// We get an error when we parse [Content_Types].xml
 			// because it's not a valid URI.
-			if (entry.getName().equals(
+			if (entry.getName().equalsIgnoreCase(
 					ContentTypeManager.CONTENT_TYPES_PART_NAME)) {
 				return null;
 			}
@@ -218,7 +218,7 @@ public final class ZipPackage extends Package {
 			// We assume we can continue, even in degraded mode ...
 			logger.log(POILogger.WARN,"Entry "
 							+ entry.getName()
-							+ " is not valid, so this part won't be add to the package.");
+							+ " is not valid, so this part won't be add to the package.", e);
 			return null;
 		}
 	}
diff --git a/src/ooxml/testcases/org/apache/poi/xssf/usermodel/TestXSSFBugs.java b/src/ooxml/testcases/org/apache/poi/xssf/usermodel/TestXSSFBugs.java
index 504eceb0ac..5a6e250d9e 100644
--- a/src/ooxml/testcases/org/apache/poi/xssf/usermodel/TestXSSFBugs.java
+++ b/src/ooxml/testcases/org/apache/poi/xssf/usermodel/TestXSSFBugs.java
@@ -397,4 +397,19 @@ public final class TestXSSFBugs extends BaseTestBugzillaIssues {
           } catch(IllegalStateException e) {}
        }
     }
+
+    /**
+     * A problem file from a non-standard source (a scientific instrument that saves its
+     * output as an .xlsx file) that have two issues:
+     * 1. The Content Type part name is lower-case:  [content_types].xml
+     * 2. The file appears to use backslashes as path separators
+     *
+     * The OPC spec tolerates both of these peculiarities, so does POI
+     */
+    public void test49609() throws Exception {
+        XSSFWorkbook wb =  XSSFTestDataSamples.openSampleWorkbook("49609.xlsx");
+        assertEquals("FAM", wb.getSheetName(0));
+        assertEquals("Cycle", wb.getSheetAt(0).getRow(0).getCell(1).getStringCellValue());
+
+    }
 }
diff --git a/test-data/spreadsheet/49609.xlsx b/test-data/spreadsheet/49609.xlsx
new file mode 100755
index 0000000000..03d9d12ca2
Binary files /dev/null and b/test-data/spreadsheet/49609.xlsx differ